Reservoirs across Australia are recording dwindling inflows as the climate warms and dries, a trend that is likely to continue and force cities, including Melbourne and Sydney, to bolster the security of water supplies.

A new study by University of NSW scientists published in the Water Resources Research journal examined streamflow data for 222 catchments and applied six of the latest climate models. All models forecast drops in supply.

Climate change is already making it harder to fill Australia's main dams, such as Warragamba, and the problems are likely to get worse, new research finds. Credit:Brendan Esposito

"We are looking at an average of 20 per cent reduced reliability in the future across all the catchments considered," said Ashish Sharma, a professor at UNSW's School of Civil and Environmental Engineering, and an author of the report.

"While this might not matter a lot up north where you have lower demands compared to inflows, this is pretty serious down south where the demands are high and we are already seeing impacts of the drought," he said.