Rainfall harvest = ( RA * AF * 0.9 )
RA - Roof Area
AF - Amount of Rainfall
0.9 - Amount of total rainwater expected for harvesting.
Adelaide's Rainfall is Unreliable
Don't forget that annual rainfall in Adelaide can range between 250-1000ml - depending on your location and natural variablity.
For more detailed information about rainfall in the Adelaide region, this page is excellent: http://www.hillsrain.com/ranges_map_colour_2.shtml