The Murray-Darling Basin in south-eastern Australia has significant ecological, social, cultural, and economic value. However, native fish populations have declined substantially in the last 50–100 years. These declines have been attributed to multiple causes, including altered flow regimes, fish barriers, exotic species, and overfishing. Given the Basin’s status as one of the world’s most regulated river basins, changes to natural flow regimes have received significant attention from researchers and policy makers. Delivery of environmental water is a key management intervention intended to address the negative impacts of flow alteration, thus restoring critical life-history processes and supporting the long-term persistence of native fish populations. We present an evaluation of fish outcomes from the use of environmental water over 7 survey years (2014–21) at 6 rivers located across the Basin as part of the Commonwealth Environmental Water Office’s Monitoring, Evaluation and Research Program (Flow-MER). Our findings indicated that environmental water provided benefits to native fish populations and supported critical life-history processes, though responses differed among years, species and rivers. The effects of environmental water on fish responses were driven by reductions in the number of low flow days and, to a lesser extent, increased average daily flows at the rivers suggesting the value of environment water may be more effective under low flows conditions. Scaling findings from river scale to Basin scale presents many challenges. We suggest predictive modelling at unmonitored sites as one way forward, and present a case study on Murray cod recruitment to demonstrate how the contribution of environmental water could be predicted at unmonitored gauges across the Basin once sufficient fish and flow data are available. Evaluating the effects of environmental water on fish populations in the Basin has led to advances in knowledge concerning fish biology, population processes and population dynamics.