Zone 2 -40 to -45.6
Zone 3 -34.4 to -40
Zone 4 -28.9 to -34.4
Zone 5 -23.3 to -28.9
Zone 6 -17.8 to -23.3
Zone 7 -12.2 to -17.8
Zone 8 -6.7 to -12.3
Zone 9 -1.1 to -6.7
Zone 10 4.4 to -1.1

Roof Area X Annual Rainfall = Maximum Available for Capture

Then determine:
Most Days without Rain X Average Daily Water Use = Storage Volume Required on Day One of a Dry Spell

Should you want more information from Rain Harvesting, here is a link to their website: http://rainharvesting.com.au/rainwater-knowledge-centre/rainwater-harvesting