In 2002, the Australian, New South Wales and Victorian governments agreed to the Snowy Water Inquiry Outcomes Implementation Deed for environmental flows to (in part) restore the health of the Snowy River in south-eastern Australia. This was the first legally binding commitment to deliver annual environmental flows in Australia. Twenty years on, we assess this Deed and its implementation to derive lessons that can inform environmental flows agreements globally. Information from governance documents, flow release data and interviews with stakeholders are used to evaluate the effectiveness of the Deed. The target of 212 GL year−1 from 2012 has not once been reached. In turn, we find that implementation has been hindered by release of too little water, overly complex institutions that lack ownership and accountability, and no provision for review of the Deed. The lessons for effective environmental flow institutions are: (a) set clear, science-based environmental restoration objectives with stakeholders; (b) make roles and responsibilities for implementation clear; (c) enable independent and transparent monitoring, reporting and regulation; and (d) undertake periodic review to incorporate new knowledge, and to adapt to climatic and other unanticipated changes.