Consumers’ capabilities, opportunities and motivations that drive single-use plastic (SUP) reduction: a qualitative study on the case of milk packaging

The rise in single-use plastic (SUP) packaging is alarming. For instance, approximately 500 billion plastic bags are used worldwide each year, equating to over 1 million bags every minute, each lasting only 15 min (Plastic Oceans, 2023). Additionally, 15,000 plastic bottles are sold per second globally, totalling 480 billion annually, with Coca-Cola selling 110 billion bottles each year (Plastic Soup Foundation, 2023). This surge in SUP consumption has resulted in significant waste and pollution, with severe environmental and health consequences. Since the 1950s, 8.3 billion metric tons of SUP have been produced, with 6.3 billion metric tons now waste (Geyer et al., 2017). Discarded plastics also pose risks to human and animal health, leading to endocrine disruption, neurological effects, and organ damage (Cook and Halden, 2020).

Current solutions mainly focus on improving recycling systems, such as enhancing waste-sorting infrastructure and educating consumers about proper disposal (Bian et al., 2022). However, large volumes of single-use plastic continue to accumulate in landfills and ecosystems, causing pollution, harming health, and endangering wildlife (Dey et al., 2021). For example, nearly 48,000 tonnes of plastic milk bottles become waste yearly, with only 5% recycled in Australia (Madden et al., 2023). This issue arises from inadequate waste management and high demand for SUP packaging, which is cheaper and lighter than reusable alternatives (Heidbreder et al., 2019; Van Hau, 2024). Therefore, addressing SUP consumption at its source is crucial for achieving long-term environmental and health benefits.

Two main approaches are commonly employed to reduce single-use plastic (SUP) consumption: individual-level (I-level) interventions and system-level (S-level) interventions (Chater and Loewenstein, 2022; Connolly et al., 2024). I-level interventions focus on altering individual consumers’ behaviours and cognitions through education, information, or other behaviour-change techniques (Heidbreder et al., 2019), and they are frequently used by behavioural scientists (Chater and Loewenstein, 2022). However, these interventions often address SUP consumption in general, ignoring the distinct ways in which consumers use specific plastic products (e.g. bottles vs. bags) in various contexts (Heidbreder et al., 2021; Mathew et al., 2024). While they show short-term success (Truelove et al., 2023), their effectiveness declines without continued funding (Loschelder et al., 2019), thereby reducing their long-term impact on SUP consumption.

S-level interventions, on the other hand, aim to restructure the choice environment itself, thereby influencing consumer decisions more directly and specifically (Chater and Loewenstein, 2022). These interventions typically employ regulatory measures (e.g. bans) (Jehangir et al., 2022) or economic instruments (e.g. taxes, levies, deposit-refund schemes) (Martinho et al., 2017), frequently employed by policymakers (Chater and Loewenstein, 2022) if policy initiatives are supported by members of the public. Policy changes can be implemented at the macro level (e.g. country or regional) or the meso level (e.g. communities, cafés, supermarkets, or local shops) by limiting or removing SUP options (Bharadwaj et al., 2021). However, S-level interventions typically focus on a limited range of items, with SUP bags being the most common target, while other prevalent SUP forms, such as milk bottles, are often overlooked (Allison et al., 2022; Borg et al., 2022). This narrow focus can lead to substitution effects, where consumers shift from banned or taxed items to other SUP products (Jehangir et al., 2022; Macintosh et al., 2020). Furthermore, the effectiveness of S-level policies heavily depends on enforcement. In areas with weaker oversight, like farmer’s markets or smaller shops, SUP consumption often remains unaffected, diminishing the potential impact of these interventions (Bharadwaj et al., 2023; Frey and Cifuentes, 2024).

Moving toward reusable non-plastic alternatives is crucial in reducing the environmental impact of single-use plastics (SUP). Reusable non-plastics, like glass, metal, or fabric, can be used multiple times, reducing the need for disposable plastic products. Recyclable plastics face challenges with contamination, sorting, and collection, limiting their effectiveness. Reusable non-plastic systems reduce plastic waste and encourage sustainable habits when designed well and accessible (Heidbreder et al., 2021). However, effective implementation requires support at both the individual (I-level) and systemic (S-level) levels (Andreas and Jabakhanji, 2023) because individual behaviours alone may not be sufficient without the structural support of policies, infrastructure, and incentives that promote sustainable practices.

Milk is consumed by an estimated 6 billion people worldwide (Food and Agriculture Organization, 2023), reflecting its status as a dietary staple across diverse cultures. Currently, there is heavy reliance on single-use plastic (SUP) bottles for milk packaging due to their convenience and cost-effectiveness for both consumers and producers (Błażejewski et al., 2021). In 2023, global dairy production reached over 950 million metric tons (Food and Agriculture Organization, 2024); if packaged in 2-L plastic bottles, over 19 million tons of bottles would be used annually. This increased production and consumption of SUP milk bottles creates significant environmental issues, including pollution, resource depletion, and carbon emissions from production and disposal.

New Zealand faces both economic opportunities and ecological challenges from large-scale dairy production. As one of the world’s largest dairy exporters, it supplies nearly 30% of internationally traded dairy products, generating NZD 18.6 billion in export revenue in 2021 (Dairy Companies Association of New Zealand, 2023). Domestic consumption is also high, with around 535 thousand metric tons of milk consumed annually (Granwal, 2024). Yet this strong demand feeds into a substantial volume of SUP waste: each year, around 23,000 tonnes of plastic milk bottles are discarded in New Zealand, a large proportion of which are neither recycled nor reused effectively (Roden, 2024; WasteMINZ, 2020). Most bottles end up in landfills (Farrelly and Green, 2020) or are exported to non-OECD countries (e.g. Malaysia, Indonesia, and Vietnam), where contaminated plastics are often burned (Farrelly and Chitaka, 2023). While SUP bottles dominate New Zealand’s milk-packaging market, several alternatives exist in smaller segments (Table 1) (Food and Agriculture Organization, 2023; WasteMINZ, 2020).

These practices highlight the limitations of current supply chains and recycling efforts, reinforcing the urgency to reduce SUP milk-bottle consumption. While recycling initiatives are often promoted, achieving meaningful circularity requires eliminating single-use plastics from the system (Mathew et al., 2023; Van Hau, 2024). Glass bottles and refill systems offer promising reusable alternatives, yet previous life-cycle assessments have shown that glass bottles can generate higher environmental impacts than both plastic and carton packaging for perishable liquids (Brock and Williams, 2020; Stefanini et al., 2021). More recently, Nilsson et al. (2024) reviewed evidence on reusable milk packaging and found limited support for environmental or safety advantages compared with recycling alternatives. Together, these studies underline the need for a balanced, evidence-based understanding of the trade-offs among packaging materials. To the best of current knowledge, most efforts have focused on improving recycling rather than developing robust “reduce and reuse” models. This study aims to set an agenda for advancing both individual- and system-level solutions to reduce SUP milk bottles in New Zealand.

Existing interventions to reduce single-use plastics (SUP) often use individual-level (i-level) or system-level (s-level) approaches to drive behaviour change. However, many efforts focus on measuring behaviour changes without analyzing underlying behavioural and attitudinal mechanisms, making it difficult to determine effective elements. As a result, i-level interventions are often temporary, while s-level tactics (e.g. bans or taxes) rely on strict enforcement. A more robust use of theory could address these issues. Evidence shows that interventions are most successful and enduring when grounded in strong theoretical frameworks (Michie et al., 2018; Rundle-Thiele et al., 2019). A solid theoretical grounding is fundamental to any effective behaviour change intervention. As Brennan et al. (2014) noted, “if the theoretical input is either incorrect, of low quality, or irrelevant to the problem at hand, the result will be ineffective regardless of the quality of the intervention” (p. 331). Indeed, interventions underpinned by appropriate theoretically-driven behaviour change models are more successful and lead to longer-lasting changes (Michie and Abraham, 2004).

Current efforts to reduce SUP packaging typically employ cognitive theories—where individuals use reason and information to make optimal choices—or conative theories, which view decisions as rooted in prior experiences and social environments (Brennan et al., 2014). Many SUP reduction interventions adopt the Theory of Planned Behaviour (TPB) (Ajzen, 1991), focussing on perceived behavioural control, subjective norms, and intention (Heidbreder and Schmitt, 2020; Heidbreder et al., 2020; Truelove et al., 2023). However, TPB predictors were found to be effective only in the initial adoption phase, with little impact on sustained behaviour (Heidbreder and Schmitt, 2020). Similarly, Heidbreder et al. (2020) found that rational choice models worked mainly for participants with low pro-environmental identities. Bertossi et al. (2024) perceived environmental benefits appear to influence purchase intention only for plastic and paper single-use cups, whereas perceived contamination risk affects only the reusable cup. These findings suggest that relying solely on individual psychological models may overlook the complexity of human behaviour (Rundle-Thiele et al., 2019), including factors related to “consumer myopia” (Brennan et al., 2016).

Conative models, such as the Nudge theory (Thaler and Sunstein, 2009), are frequently employed to alter the choice environment so consumers are prompted to change their behaviour (Homonoff, 2018; Mundt et al., 2020). Mundt et al. (2020) found that nudges facilitate behaviour shifts without educating consumers on why changes are necessary, undermining the development of lasting habits. For example, banning SUP bags may reduce their use but lead to increased consumption of other plastic products, such as bin liners and produce bags (Frey and Cifuentes, 2024; Macintosh et al., 2020). Psychological insights are needed to help consumers understand the rationale behind these changes, improving sustained adoption (Rundle-Thiele et al., 2021). Many of the theories mentioned overlook the complexity of adopting a systems approach (Rundle-Thiele et al., 2019), leading to an intention-behaviour gap (ElHaffar et al., 2020). This suggests that a comprehensive model addressing both individual and system aspects should be applied to SUP reduction.

The Capability–Opportunity–Motivation–Behaviour (COM-B) theory offers a valuable framework for identifying the individual, sociocultural, and situational factors that influence behaviour (Michie et al., 2011), combining both conative and cognitive models. According to the model, individuals must have both the physical and psychological ability (C), sufficient opportunity (O), and adequate motivation (M) to perform a given behaviour (B) at any moment (Michie et al., 2014). In the COM-B model, capability refers to an individual’s knowledge, skills, and abilities to engage in a particular behaviour in COM-B, which encompasses psychological capability (mental state and level of knowledge), and physical capability. Opportunity encompasses external factors facilitating behaviour execution, including physical and social opportunities. Motivation refers to the internal processes that impact decision-making and behaviour. The two primary components of motivation are extrinsic motivation, driven by external rewards or pressures, and intrinsic motivation, driven by personal interest, enjoyment, or internal satisfaction. Previous research and use of the COM-B framework have effectively identified perceived barriers linked to the capability, opportunity, and motivation components (Allison et al., 2021). In addition, COM-B has supported the creation of integrated marketing campaigns designed to promote behavioural change (Manika et al., 2022). Therefore, this theoretical perspective for a behavioural diagnosis pinpoints the COM-B elements that inform design interventions.

The COM-B model has also been used in other relevant pro-environmental literature, for instance, reducing food waste (Manika et al., 2022) and encouraging recycling behaviours (Roy et al., 2022), while its application in the domain of curbing plastic packaging use remains relatively scarce (Mathew et al., 2023). This exploratory study addresses that gap by leveraging the COM-B model to investigate the capabilities, opportunities, and motivations that shape the reduction of single-use plastic (SUP) milk bottle consumption among New Zealand residents who, at the time of data collection, were partially reducing their SUP dependence or had not yet adopted any reduction strategies. Drawing on existing SUP reduction literature and the COM-B framework, this research further illuminates the barriers and facilitators to change and provides actionable insights for designing interventions. Consequently, the following research questions (RQs) were identified to guide the study:

This study employed an interpretive approach to explore the drivers and barriers to reducing the reliance on SUP milk bottles. In-depth interviews were used to test the COM-B theory in the context of reducing SUP milk packaging in New Zealand. Qualitative research design offers a thorough exploration of complex phenomena (Saunders et al., 2019). Qualitative approaches, such as in-depth interviews, can capture insights into consumers’ perspectives that might be overlooked by alternative research methods, especially the topics that have not been explored before (e.g. plastic milk bottles).

Purposive sampling was used to recruit participants with substantial knowledge of the research topic (Creswell and Clark, 2018; Saunders et al., 2019). Eligible participants were consumers who had reduced their use of single-use plastic (SUP) milk bottles, intended to do so or had contemplated doing so and consumed milk at least once a week. All participants were over 18 and resided in New Zealand. A brief screening questionnaire was distributed to confirm interview eligibility.

A total of 25 participants were recruited through advertisements in three New Zealand cities (Dunedin, Auckland, and Wellington) and via social media platforms, including the authors’ personal and professional networks. Five individuals were excluded for the following reasons: (1) they did not consume milk at all, or (2) they purchased only 1 litre of milk per month. The remaining 20 eligible participants were invited to an interview, with informed consent obtained via email before scheduling.

Appendix 1 in the Supplementary file presents the demographic details of the eligible participants. In total, the sample comprised 20 participants (n = 20): 16 females (80%) and 4 males (20%), aged 21–64 years, with 9 (45%) above 40, 5 (25%) above 50, and 4 (20%) above 60. Twelve participants (60%) resided in family households, while 8 (40%) lived in flatshares. Weekly milk consumption varied from 500 mL to 10 L, with 12 participants (60%) reporting 2 L/week or more, equating to 2–3 plastic bottles per week.

All interviews were conducted between 30 August 2024 and 2 October 2024 using Zoom or Microsoft Teams and lasted approximately 20–75 min. During the interviews, participants answered questions about their capabilities, motivation, and opportunities to participate in SUP reduction following the interview protocol (see Appendix 2, Supplementary file). The interview guide was developed by existing research on behaviour change, specifically drawing on the COM-B framework (Manika et al., 2022; Roy et al., 2022).

All interviews were audio-recorded to accurately transcribe data, ensuring precise capture of participants’ responses by the lead author. The recordings were transcribed verbatim, excluding non-verbal cues (e.g. pauses, laughter), and personal identifiers were removed to maintain confidentiality. Transcripts were imported into NVivo 14 for analysis.

Data analysis was conducted using reflexive thematic analysis step by step (Braun and Clarke, 2006), following a deductive approach informed by the COM-B model (Michie et al., 2011) (see Appendix 3 in Supplementary file). A preliminary coding book (see Appendix 4 in Supplementary file) was developed using the COM-B categories—capabilities, opportunities, and motivation. The transcripts were coded to reduce single-use plastic (SUP) milk bottles, with sub-categories created inductively within each category. For instance, under the “capabilities” category (barriers), subcodes were created for “Practicality and Durability of Plastic” and “Required skills in using reusable glass bottles” (see Appendix 5 in Supplementary file for details of codes and themes).

These codes were mapped into themes, as summarized in Figure 1. The first researcher coded all transcripts and proposed an initial set of themes; another researcher independently coded 25% of the transcripts (Cohen’s kappa = 0.83). The research team collectively reviewed and refined the themes to ensure consistency and accuracy.

Enablers and Barriers to reducing SUP milk bottle consumption in the New Zealand context were identified through reflexive thematic analysis. The findings are organized into barriers and enablers with respect to the capability, opportunity and motivation components of the COM-B model. Twenty-one key themes emerged from the codes, which were mapped to the COM-B model’s Capability, Opportunity, and Motivation components (as depicted in Figure 1).

Capability is the factor that helps people use their knowledge, skills, or abilities to act (Section 2.2). Regarding the enablers of capability, four key themes emerged from the data. The first theme, “awareness of the plastic waste problem” (CE1), underscores that many consumers recognize how plastic waste contributes to pollution, climate change, and global warming. For instance, Participant 4 noted:

The second theme, “complexities of proper recycling practices” (CE2), highlights the considerable skills and knowledge gap consumers face when properly recycling plastic. Tasks such as identifying different types of recyclable plastics, rinsing containers, and removing non-recyclable lids can be challenging. Participant 14 expressed frustration in this regard:

The third theme, “awareness of downcycling limitations” (CE3), points to participants’ understanding that plastic recycling often results in downcycling, wherein recovered plastic becomes lower-quality material unsuitable for reuse in food packaging. Participant 8 questioned the broader environmental impact of recycling:

The fourth theme, “growing recognition of glass as a functional and sustainable alternative” (CE4), arises from participants’ awareness of glass’s reusability, lack of microplastic contamination, and effectiveness in keeping milk colder and fresher for longer. The availability of milk in glass packaging reinforces consumers’ capacity to make more sustainable choices.

The last theme is “awareness of milk in plastic linked to food waste” (CE5), highlighting that milk in large plastic containers could become waste. Participant 7 highlighted how large plastic containers can lead to milk waste when the product spoils before it can be finished:

Regarding the barriers to capability, two key themes emerged from the data. The first theme, “practicality and durability of plastic” (CB1), highlights plastic’s capacity to remain intact under pressure and its extended shelf life, both of which make it an appealing choice for consumers prioritizing convenience and safety. Participant 11 illustrated this point:

The second theme, “required skills in using reusable glass bottles” (CB2), shows the logistical and cognitive challenges of adopting milk glass bottles. Consumers must remember to wash and return the bottles, which is less convenient than using single-use plastic. Participant 2 explained:

Opportunity refers to external conditions or resources that make it easier to perform a behaviour, such as access to tools (physical opportunity) or support from others (social opportunity) (Section 2.2). In terms of enablers of opportunity, only one theme emerged from the data: “cultural shift towards sustainability” (OE). This theme captures a growing societal awareness and adoption of sustainable practices across various levels, encompassing recycling programs, school educational campaigns, and public advocacy to reduce plastic consumption. Efforts to promote alternatives to single-use plastics are increasingly coupled with emotive campaigns linking environmental issues to recognizable symbols. Participant 7 mentioned the global initiative:

Related to barriers of opportunity, seven themes emerged from the dataset. Collectively, these themes illustrate the multifaceted challenges consumers face when transitioning from single-use plastic to more sustainable alternatives. The most prominent theme is “limited access to reusable packaging options” (OB1), highlighting the scarcity of reusable alternatives—particularly glass bottles—in mainstream retail outlets such as supermarkets. As Participant 2 described:

The second theme is “the dominance of plastic packaging in the market” (OB2), reflecting the perception of plastic as the default, most accessible choice. Participant 6 voiced a concern over the omnipresence of plastic:

The third theme is the “perceived limitation of current reusable systems” (OB3), which emphasizes the logistical and practical difficulties consumers encounter when trying to use reusable bottles. Participant 20 explained:

The fourth theme is “lack of consumer awareness and education” (OB4), underscoring the importance of informed consumer choices and targeted outreach. Participant 13 noted that while broader environmental campaigns exist, they seldom focus on milk:

The fifth theme is “insufficient policy and regulatory support” (OB5), which focuses on government measures that could facilitate the transition to sustainable milk packaging. Participant 4 observed:

Participant 17 suggested that price interventions and incentives could help overcome cost barriers:

The sixth theme is “the absence of milk delivery services or refill stations” (OB6), drawing attention to the decline of once-widespread systems that deliver milk directly to consumers in reusable glass bottles. Participant 18 reflected on these earlier practices:

The last theme is the “lack of diverse options in reusable alternatives” (OB7), the limited range of products offered in reusable formats. Although some stores occasionally stock dairy milk in glass bottles, non-dairy and speciality milk remain largely unavailable in such formats. Participant 9 commented:

Participant 16 echoed this sentiment:

Motivation—internal drivers of behaviour—includes extrinsic (external rewards/pressures) and intrinsic (personal interest/satisfaction) (Section 2.2). Four themes emerged from the data regarding enablers of motivation to reduce SUP milk bottles. The first theme is “growing preferences for the quality and taste of glass-bottled milk” (ME1), focussing on consumers’ perception that milk in glass bottles tastes better and is of higher quality. This preference is intrinsic and rooted in personal satisfaction and enjoyment. Participant 1 emphasized these benefits:

The second theme is “concern about the health and safety of plastics” (ME2), with many participants favouring glass over plastic to avoid potential chemical leaching and microplastic contamination. Participant 10 explained:

The third theme is “support for local and small business” (ME3), reflecting participants’ desire to contribute to the local economy and support ethical, small-scale producers. Many respondents described deliberately avoiding large corporations that heavily rely on plastic. Participant 4 said:

The fourth theme is “growing commitment to developing sustainable non-plastic habits” (ME4), which captures participants’ efforts to cultivate long-term habits that reduce plastic use. Many described organizational strategies and routines to facilitate glass bottle usage. Participant 6 remarked:

The first theme, “affordability of plastic bottles” (MB1), highlights how plastic packaging is often cheaper and more readily available—an essential factor for consumers on tight budgets. As Participant 20 explained:

The second theme, “convenience of plastic bottles” (MB2), reflects the lightweight, durable, and accessible nature of plastic. These attributes make it practical for consumers who prioritize ease of use and minimal breakage risk. Participant 10 offered a telling perspective:

This study applies the Capability-Opportunity-Motivation-Behaviour (COM-B) model to explore how single-use plastic (SUP) milk bottle consumption can be reduced. While the model’s capability, opportunity, and motivation elements help categorize factors, the results reveal its limits when system-level forces are not taken into account. Awareness of plastic’s environmental harm and recognition of glass as an alternative (capability) do little to change behaviours when market structures and habits favour plastic. These findings challenge assumptions that knowledge and good intentions alone reduce plastic consumption (Mundt et al., 2020), aligning with Otto et al. (2021), who stress that awareness and eco-labelling must be supported by structural enablers. Our findings also parallel recent evidence that effective sustainable-consumption interventions must operate across both individual (I-) and system (S-) levels (Van Hau et al., 2025a).

Opportunity factors highlight the interplay between cultural and structural elements. Societal efforts such as educational campaigns can increase consumers’ willingness to switch to alternative packaging, yet social norms alone cannot overcome the physical and economic barriers embedded in the marketplace. Limited availability of reusable options, higher upfront costs, and the dominance of plastic in supply chains exemplify how system constraints undermine even strong consumer motivation (Mathew et al., 2024; Truelove et al., 2023). Consistent with life-cycle and behavioural evidence (Brock and Williams, 2020; Nilsson et al., 2024; Stefanini et al., 2021), these results show that structural enablers must complement cultural messaging to make sustainable packaging practical and affordable.

Finally, while COM-B stresses the interplay between reflective (e.g. environmental values) and automatic (e.g. habit, convenience) motivations, our results reveal that these forces often conflict in everyday contexts. Similar to prior work on sustainable behaviour (Barkela et al., 2021; Bharadwaj et al., 2021), participants with strong environmental values frequently prioritized convenience and cost, underscoring the persistence of “friction costs” that suppress pro-environmental action. This gap between attitudes and behaviour highlights the need for behaviour-change models to incorporate the reinforcing relationship between system design and motivation, where convenience and affordability are not merely personal preferences but reflections of structural opportunity (Carrigan et al., 2023). Overall, this study extends COM-B by illustrating how the transformation of SUP habits requires coordinated changes across capability, opportunity, and motivation at both I-level and S-level, turning “green” intentions into a routine of sustainable consumption.

On a practical level, this study offers several insights that could help policymakers, retailers, and community organizations encourage consumers to reduce SUP milk bottles. Opportunity barriers highlight the need for infrastructural improvements and policy interventions. Many participants indicated they would switch to better reusable alternatives if they were more conveniently available at supermarkets or offered through affordable delivery services, supporting findings from Hallez et al. (2024). Promoting deposit-return systems, where consumers receive refunds or discounts for returning bottles, could reduce cost and inconvenience. However, viable alternatives must be cost-effective, especially for high-volume products like milk. Therefore, an S-level intervention mandating affordable reusable provision could encourage consumers to adopt reusable packaging and challenge the perception of higher costs.

This study found that motivational drivers such as better taste, aversion to microplastics, and support for small local producers encouraged behaviour change toward sustainable milk consumption. Branding and marketing that emphasize these sensory, ethical, and environmental benefits can motivate consumers to adopt more sustainable purchasing habits (Hamlin, 2016). Participants perceived reusable glass-bottled milk as fresher, healthier, and more aligned with their personal values. They also noted that buying milk in large plastic containers often led to food waste, suggesting that promoting smaller alternative sizes could help reduce both waste and plastic use. Additionally, participants noted the limited availability of plant-based milk options in reusable or refillable packaging. This presents an opportunity for producers to develop sustainable solutions that address both packaging and nutritional aspects, especially as plant-based alternatives with comparable nutritional profiles to dairy milk continue to gain global demand (Reyes-Jurado et al., 2023; Van Selm et al., 2022).

Practicality remains a key barrier, as plastic is widely perceived as cheaper, lighter, and more durable than glass (Parsons, 2022). To reduce reliance on plastic, alternative packaging must be both visible and price-competitive in the market. However, consumer perceptions of safety and freshness often reflect misconceptions rather than scientific evidence (Boesen et al., 2019; Lindh et al., 2016). For instance, while participants viewed glass bottles as safer, fresher, and more environmentally friendly, research shows that microplastics can also occur in glass and carton-based packaging (Kutralam-Muniasamy et al., 2020; Schymanski et al., 2018), and can be released from plastic caps or during cleaning processes (Winkler et al., 2019). Evidence also indicates that microplastics have been detected in milk products themselves, suggesting broader systemic contamination beyond packaging (Zhang et al., 2023). The environmental impact of glass may further be higher due to its heavier weight and energy-intensive production (Boesen et al., 2019). These findings highlight the need for clearer communication and eco-labelling to bridge the gap between consumer perception and scientific evidence. Beyond glass, retailers could explore steel containers, carton packaging, refill stations, vending machines, or milk delivery systems to promote waste reduction. However, these initiatives require retailers’ commitment to invest in infrastructure that supports sustainable supply systems. Complementary policy measures such as levies on plastic bottles, tax incentives for dairies, and phased reductions in plastic production could further stimulate industry innovation and long-term transition toward circular packaging models (Korhonen et al., 2015).

This study has several limitations, each presenting opportunities for further research. First, as it was conducted in New Zealand, future research should explore other contexts, including developing countries and different products (e.g. bottled water), to provide a broader perspective. Second, alternative methods, such as co-design, surveys, or experiments, could offer deeper insights. Third, the small sample size of 20 participants could be addressed by incorporating focus groups or co-design sessions to capture a broader range of segmentation (Van Hau et al., 2025b). Fourth, although this study applied the COM-B framework effectively, the interview guide was intentionally kept succinct to ensure participant engagement and manage time constraints. As a result, some sub-dimensions within each COM-B component were not fully explored. For instance, additional Capability questions (e.g. confidence, training, or physical ability), Opportunity prompts (e.g. specific social or policy influences), and Motivation items (e.g. goals or emotional drivers) could further enrich understanding in future research. Similarly, while Behaviour was the focal outcome in this study, future studies could explicitly operationalize and measure behavioural frequency, context, or steps to provide greater behavioural specificity. Lastly, while the study focused on plastic milk bottles, future studies could expand to other SUP types, such as straws or meat trays, especially those relevant to local contexts.

This study extends the understanding of single-use plastic (SUP) reduction by focussing on milk packaging, an overlooked yet high-impact category. Using the COM-B framework, it highlights how individual capabilities, motivations, and system-level opportunities interact to shape sustainable choices. The findings reveal that effective change requires aligning behavioural interventions with supportive infrastructure, pricing, and policy. While reusable options like glass can help, their benefits depend on local reuse rates and logistics, reinforcing the need for holistic, life-cycle-based strategies. Broader sustainability progress will depend on integrating behavioural, economic, and environmental perspectives in both policy and practice.

This research was reviewed and approved by the Human Ethics Committee, University of Otago (Reference number: 24/0396). Informed consent was obtained from participants before the commencement of the study.

The supplementary material for this article can be found online.