This article examines interactions between different forms of authoritative knowledge and evidence in a public dispute over an environmental problem. It draws on a case set in a small coastal town in New Zealand where the local community had expressed concern over the degradation of a river-mouth estuary caused by catchment management works built in the 1950s to support the farming sector. The estuary historically had been an important economic and cultural treasure for Indigenous Māori, and by the mid-20th century had become a valued recreational and fishing resource for the broader community. This article analyses a moment of dispute in the 1980s between those who called for the restoration of the estuary and those who wished to maintain the status quo. Drawing on an analysis of official reports, media coverage and other public documents, the article shows how the competing parties and their constructions of the collective good accorded authority and weight to specific histories, forms of evidence and kinds of people. The article understands the case not as a dispute between “the people” and “the experts” but rather as a moment where competing blocs drew on specific grammars of justification in their attempts to align their claims with the collective good.