It's our biggest river system by a wide margin, where most of our agricultural (and wine) land is. Water quality, water access, salinity and ecological damage have been a huge issue for decades due to the fragmented governance of the area (Queensland, New South Wales, Victoria and South Australia all have a big interest in the basin).
It boils down to: 11.5 of 13 GL of divertable flow and 24 GL average total flow are removed for different uses. About 95% is agricultural use including rice and cotton. That much removal leaves way less than enough for the environmental requirements to maintain the health of the rivers.