Do you have a soil report for your block? If yay then use that as a guide. Otherwise consult your local council and enquire about soil types in your area. have a look at this site.
http://www.build.com.au/building-reactive-soil-sites
My site is only slightly reactive, and top soil is shallow. My footing will only be 600 x 600 x 700 deep. because it can be.