Successful integrated water resource management (IWRM) is predicated on increased stakeholder participation and achieving environmental outcomes by locating decision‐making at the catchment scale of management. However, geographic scale impacts opportunities for participation and privileges some actors within the functional and spatial scope of decision‐making. Accordingly, rescaling may merely reconfigure rather than redistribute power. With its dual national and regional management of freshwater based on river catchments, New Zealand's experience of integrated environmental management provides insights for wider theory and practice. In response to recent significant freshwater quality decline from agricultural non‐point discharges over the last two decades and related public concern, the national government and regional councils have redefined participation constellations at national and regional government levels. This paper compares stakeholder diversity and policy effectiveness of regulatory agencies and collaborative initiatives at both national and regional levels, using the Manawatu River catchment as a case study. It detects low participator diversity within regulatory agencies, but relatively high policy salience. In contrast, the collaborative forums' low policy salience, despite their high participator diversity, raises questions about the substantive value of non‐regulatory collaborative initiatives. Copyright © 2016 John Wiley & Sons, Ltd and ERP Environment