Formula:

Rainfall harvest = ( RA * AF * 0.9 )
Where,
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