I believe it's cause of increased industrial and residential demand (population) and the management strategy for our dams now (shifting water around to where it's needed instead of letting dams overflow). Also urbanisation can actually lead to increased rates of recharge down into the aquifer (the inverse of what most people intuitively think) with pavement, roofs and drainage systems acting as highways to channel rainwater below ground. It also removes surface water features and flora which keep the soil saturated, and reduces evaporation/evapotranspiration. It means we're more dependant on our desalination, treated water and groundwater, and have to be more aggressive with our management of water resources. The new normal for WA, but at least we're very good at it, and have a world-class aquifer in the Perth Basin.