Shifting political alliances and new environmental challenges are prompting debate over processes of sub-regionalisation and whether the interplay between multiple scales of governance leads to positive synergistic outcomes or negative institutional disruption. Regional management of tuna fisheries in the Western and Central Pacific Ocean is an example where a web of treaties, conventions and institutional frameworks underlie international cooperation. Through examining the interplay between the regional Western and Central Pacific Fisheries Commission and sub-regional Parties to the Nauru Agreement, this paper explores the extent to which the PNA and WCPFC interact in the management of regional tuna fisheries. The results demonstrate that for contested marine resources such as fisheries, international sub-regions can go beyond functional units to also present wider opportunities to shift power relations in favour of small island states. Additionally, the presence of sub-regional groups like the PNA has served to challenge the performance of the WCPFC, stimulating greater debate and progress within the regional body. The paper concludes that the combined work of the PNA and the WCPFC puts them ahead on many issues and may represent a testing ground for a functional multilateralism utilising both regional and sub-regional governance platforms for the management of shared resources.