This paper positions legitimacy and trust within a post-colonial theoretical frame, challenging the fundamentals of Australia’s water governance system as well as the presumptions of neutrality that underpin liberal water management principles of participation and inclusion. In a settler colonial society like Australia that until very recently excluded Indigenous people from all forms of water governance, there are significant questions to be asked about legitimacy and trust in its water regulatory regimes, guiding policy directions and the fairness of the outcomes generated by its institutions. The paper describes attempts to build cross-cultural collaborative research and management partnerships in the environmental water sector and points to formal agreements as a mechanism through which parties, including governments, can negotiate rules governing legitimacy. As an expression of self-determination and recognition of the legitimacy of Indigenous modes of governance, agreements represent a marked improvement on the exclusionary legal, policy and knowledge-production processes that have shaped our current arrangements.