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Timing of delivery of environmental water is essential for restoring and conserving riverine ecosystems. However, change in the timing of occurrence of streamflow and its implications on environmental water allocation and river ecosystems are often ignored in current environmental water research and management. We adopted the centre of timing approach to analyse the change in the occurrence of streamflow due to climate change (through a change in rainfall) and river operations (flow regulation and diversion) in the Goulburn-Broken catchment, Victoria, Australia. It was found that annual streamflow in the catchment declined by an average of 47%, while centre of timing increased by an average of 36%. These changes were 52% attributable to rainfall decline and 48% to river regulation. A decline in annual streamflow (35%) and an increase in centre of timing (21%) across the Goulburn River would be observed by 2050 as an impact of climate change. Current management efforts should be directed at reversing the change in centre of timing to support the sustainable management of riverine systems. The Goulburn-Broken catchment is typical of managed catchments in many areas of the world with forecast rainfall reduction, and the findings are likely to be widely applicable.