In the face of continued environmental degradation, policy makers need to accelerate public uptake of pro-environmental behaviours. Promoting behaviours which catalyse the adoption of other similar behaviours through the spillover effect has been proposed as a potential solution. This requires understanding which behaviours are seen as similar and what criteria are used to identify behavioural similarity. We used a sorting procedure with 32 householders in Melbourne, Australia, to investigate the perceived similarity of household water conservation behaviours and identify the underlying constructs used to distinguish between similar and dissimilar behaviours. Location was the primary attribute used to define behavioural similarity, specifically whether behaviours took place indoors or outdoors. Participants also distinguished between curtailment, efficiency and maintenance-type behaviours. Our findings provide empirical support for existing theoretical behaviour taxonomies. The results could inform design of future water-saving campaigns to promote catalytic behaviours, by leveraging off similar, existing behaviours for effective behaviour change results.