The aim of the present study was to develop a model capable of statistically downscaling monthly GCM outputs to catchment scale monthly streamflows, accounting for the climate change. The current study investigated only the calibration and validation of the abovementioned statistical downscaling model. This was demonstrated through a case study applied to the GWMWater supply system in north-western Victoria, Australia. It is a large scale complex multi-reservoir system that is operated to meet a range of economic, social, and environmental interests.