Water is a critical resource to society for many reasons. Given the fact that water is an indispensable part of human survival and economic development, conserving it is important to ensure a sustainable supply. Water conservation has been a practice for millennia, with increasing recognition of the value of water to humans, in particular, for the environmental, economic and social goods and services it provides. Despite an increase in strategies for water conservation, achieving sustainability is still a goal and there are few guidelines that outline sustainable measures of water conservation to manage water resources against scarcity, climate change, and unpredictable environments where demand for water is increasing. Behavioural change and mechanisms of behavioural change are increasingly advocated for sustainable water-conservation behaviour in order to improve our understanding of households' attitudes and behaviour towards water resources.<br/> The aim of this PhD thesis is to investigate mechanisms of water-conservation behaviour through capability, opportunity, and motivation-behaviour (COM-B system) lenses. This thesis proposes a behavioural approach to understand water conservation, and is interdisciplinary because it brings together psychology and water conservation. This approach helps to improve our understanding of behavioural change and mechanisms of behavioural change for effective water conservation strategies. Such an approach focuses on the understanding of households' attitudes and behaviours, and the linkages between behaviour mechanisms and relevant interventions that bring about behavioural change. To do this, the importance of understanding behavioural change and ways of linking behaviour mechanisms to relevant intervention functions and policy categories were studied in three papers: 1) "Household water use and conservation behaviour: A meta-analysis; 2) "Barriers and drivers of household water conservation behaviour: A profiling approach; and, 3) "Reducing household water-use: The influence of water-conservation messages on intentions to act".<br/> The approach taken can advance the capacity-building and empowerment of water users to recognise, analyse and improve water conservation consciously and voluntarily to bring about sustainability. The study design and techniques employed in this thesis, the COM-B system and Behaviour Change Wheel (BCW) model, are an integrative framework of behaviour. These models have never been used to study water conservation and they bring a new, explicit perspective; they are best at linking attitudes and behaviour to behaviour change interventions.<br/> The first paper examines the capability, opportunity, and motivation of households, as important psychological-social dimensions influencing water-conservation behaviour. A meta-analysis of the causal mechanisms of water-use behaviour was used to identify 88 correlation points from a combined sample of 15,656 participants to show how psychological-social predictors based on capability, opportunity and motivation-behaviour (COM-B) impact household water-use reduction. Results of this paper demonstrated that the behaviour mechanisms – capability, opportunity, and motivation dimensions can lead to behavioural change. These behaviour mechanisms correlate with household water-use, with opportunity being the most important mechanism. The study also found that within each dimension, correlations differed by household socio-demographic status and actual water-conservation behaviour.<br/> The second paper identifies main drivers and barriers to household water-conservation behaviour and the manner and degree to which these drivers and barriers affect households' capability, opportunity, and motivation in water-conservation strategies. A latent profile analysis was used within the capability-, opportunity-, and motivation-behaviour (COM-B) framework to identify key barriers and drivers of household water-conservation behaviours. Participants (N = 510, mean age = 56.08 years, SD = 14.71) completed measures of psychological-social constructs related to barriers and drivers of water-conservation behaviour. A latent profile analysis yielded a 3-profile solution in which capability (35.8%), opportunity (23.2%), and motivation (41.0%) conceptualised barriers and drivers of water-conservation behaviour. Results from the profiling groups demonstrate the reoccurring barriers and drivers inhibiting and exhibiting water conservation. Major identified barriers and drivers associated with these profile groupings were time constraints, acuity of water-efficient devices, lack of skills to adopt conservation practices, and availability of incentives/disincentives for water-saving devices. The profiling approach to understand the barriers and drivers of household water conservation in relation to behaviour change theory - the Behaviour Change Wheel (BCW) framework – influenced component parts of an intervention or strategy for effective conservation.<br/> The third paper examines the extent to which receiving water-conservation messages could influence households' water scarcity concern and intentions to act in conserving water and tested whether this relationship was mediated via increasing capability, opportunity, and motivation. Participants completed a questionnaire assessing the extent to which receiving the water-conservation message types could influence their water scarcity concern and intentions to act in conserving water. Videos containing the water-conservation message types were then shown to participants. The effect of water-conservation message videos was assessed using the capability, opportunity, and motivation dimensions as mediating variables on water scarcity concern and intentions to act in conserving water. Pathways analysis (mediation analysis) was used as a tool for assessing these effects of the message types. The study found that specific messages about conservation strategies influence households' cognitive, affective, and behavioural responses to water conservation, perceived water scarcity concern, and intentions to act. Results from this study demonstrate the COM-B behaviour information can improve water-conservation activities by linking existing strategies to support water-conservation behaviour conditions to reduce vulnerability to environmental risks – including water crises.<br/> The BCW and the COM-B framework of behaviour change theory are introduced to help water conservation interventions play a more prominent role in designing strategies for sustainable water-conservation behaviours. Ways for implementing and designing such a new paradigm shift are proposed and analysed by linking behaviour mechanisms to relevant intervention functions and policy categories. This has implications for understanding the profiles of households' water-conservation behaviour and associated barriers and drivers when designing intervention strategies for water conservation. This PhD thesis analyses causal mechanisms of behaviour – capability, opportunity, and motivation to provide critical insights into sources of behaviour and how these sources combine in multiple dimensions to jointly affect water-conservation behaviour for advancing our understanding of attitudes and behaviour to achieve sustainable water conservation. It is hypothesised from the research that behaviour mechanisms in the Behaviour Change Wheel framework actively influence water conservation. In addition to its contributions to the field of interdisciplinary study of water conservation, this PhD thesis also offers recommendations for government water agencies, stakeholders, and policy-makers to build intervention strategies by promoting and understanding individual behavioural change. The Command-and-control strategies to water conservation are not achieving sustainability and bridging the behavioural approach to the demand side of water supply can achieve sustainability because it increases consumers' intentions to act when individual behaviour mechanisms are considered.<br/>