Actually 1000 gallon tanks would make a huge difference. A 5000 tank here in SE Qld. (Much dryer than Sydney) is ample for a couple all year round - total self sufficiency.
Get rid of the lawn - a rediculous notion imported from pommy land (never see that in Asia!) sucks water. Think of all the time saved from NOT mowing.
Still reckon it's cheaper to catch it save it than building nuclear power plants though...
BTW even a 10% reduction in useage would be more water than could be desalinated. Should be easy eh?