Oyster farmers depend on good water quality. Activities upstream from estuaries result in externalities that impact water quality. Over the last 10 years, oyster farmers have been developing estuary-wide environmental management systems (EMSs) to tackle internal (i.e. industry-related) and external (i.e. catchment) issues in N ew S outh W ales, A ustralia. Drawing on interview-based research and document analyses, this paper shows that the process of creating an EMS for the oyster industry, as well as the creation of the EMS itself, resulted in legitimizing the industry's stewardship over the natural resource it depends on (water). For the oyster industry, this result was due to a change in the scale on which EMSs have been developed: instead of viewing issues at the individual business level, the systems expanded their viewpoint to the entire catchment, and included every oyster business in the estuary, as well as all other activities in the upper catchment. By providing a means of communicating internal efforts and with the support of local government bodies, EMSs provided a mechanism with which influence over upstream actors and activities could be exerted. We demonstrate this by using the 'social-ecological systems' and ' ENCORE ' frameworks, emphasizing the transitions that allowed for this change of scale to take place.