In reply to Graeme Hammond:
> I thought the point was that they filled over the course of a winter when there is more rain so that there is enough water during the summer when there is less rainfall months to keep both the river running and for human consumption.
That's one purpose.
Sometimes they're used for a different purpose - to provide a buffer to hold then more gradually release storm surges, to protect downstream lands from flooding. This clearly needs them to be less than full.
Regardless, we've seen exceptionally low levels across most of the country in rivers, reservoirs and groundwater after an exceptionally dry year.
If the reservoirs exit winter 22/23 lower than they exited winter 21/22 (a distinct possibility) and we have another dry year, things start to look a bit hairy.