The rules governing the restraint of children fall under the Australian Road Rules (thus, Australia wide). However you are best to check with your State police re any specific regulations which may state otherwise. In SA, there are no rules which state where in the vehicle (other than not in the front row of seats in a vehicle other than a utility).

Pretty Butterfly is right on the mark with her comments though - good post / info PB! .

We'll be installing the third car seat in the rear of our 'family' vehicle soon too, and playing with scenario's re - where to seat each child; we may end up switching the seats around a few times until we find what works for us.