Recent controversy has highlighted the need for programs such as the Murray-Darling Basin Plan to be underpinned by robust science, and the ability of scientists to set environmental water requirements has been queried by some commentators. We developed a bespoke environmental water requirement for the terminus of the Murray-Darling Basin, the Coorong, Lower Lakes and Murray Mouth (CLLMM), to guide environmental planning. The goal for the region, to be a healthy, productive, and resilient wetland of international importance, was distilled into a suite of ecological objectives in collaboration with river managers. We used a suite of indicators including vegetation, macroinvertebrates, fish and ecological processes to represent these objectives holistically. Salinity in Lake Alexandrina was the most-flow responsive variable affecting indicator taxa and processes. Thresholds of a long-term average of 700 µS/cm electrical conductivity, maximum of 1000 µS/cm EC in 95% of years and 1500 µS/cm EC in 100% of years were set. Three-year sets of rules for minimum barrage flows were developed to support each of those targets (i.e. average annual flows of 4000, 2000 and 1000 GL, respectively). Flow in any single year could fall below these averages, but then additional flow would be required within the three-year timeframe. These flow rules were sufficient to maintain Coorong hydrodynamics and ecosystem states, even under climate change. Flows to meet the 1500 µs/cm EC target alone were not sufficient in very dry periods to prevent ecological degradation and increasing volumes of additional water could be required under climate change. Additional high-flow requirements were also needed for the Coorong (i.e. >6000 GL required with a return interval of 1:3 and >10,000 GL at 1:7, on average). These environmental flow requirements have been tested extensively and compared with EWRs developed using alternative methods and they appear robust and sufficient to maintain ecological character.