In 2017 five years will have swum by since implementation of one of Australia’s most significant and controversial water reform policies, the Murray-Darling Basin Plan. This milestone brings expectation of an evaluation, which if done robustly will ease anxieties and demonstrate the value of the policy as well as identify knowledge gaps and adaptive management opportunities. However, evaluating how the Basin Plan is contributing to sustainable outcomes for native fish populations on a Basin wide scale involves numerous complexities and requires much more than just analysis of monitoring data against Basin Plan targets for fish. Life history requirements, factors external to water reform, ecological time lags and spatial variability are all being considered when teasing out how the Basin Plan is making a difference. Understanding the level of confidence in the relationship between flow and life history requirements and the likelihood and potential impact of external factors is critical, as is an understanding of the management actions that have taken place as a direct result of implementation of the Plan. Thus, this evaluation is a challenging yet exciting space where an understanding of science and management actions must intersect to ensure that the evaluation is robust and meaningful to a public audience.