A little more than two years ago, we were worried whether our dams would run out - you can see some pretty graphs here.
Disturbingly, when the dam was finally full again after 8 years of drought in October, the state opposition leader John-Paul Langbroek called to increase the water storage level at the expense of flood mitigation. The main dam (Wivenhoe Dam) can hold 225% of it's nominal capacity for flood storage. It's currently at 190%.
The dam is a earth embankment dam and is not design to spill. If so, it may erode the dam and potentially cause it's failure. Hence, there must be a controlled release, even while the flood conditions are occuring and it's a fine balancing act between holding back more rain and flooding downstream.
In general, it is considered that the flood mitigation capacity (about that of Sydney Harbour) will knock about 2m off a flood peak. There would be many more people currently swimming without it, even before it's expected to peak in about 36 hours.