Ah well if it looks like italy at least you tried!
I think the best way to do it is how they make shped kids cakes, so make you basic cake recipe in different shaped tins, then carve up the cake afterwards. I'd prob use two round ones and perhaps a rectagula one in the middle. You can make Cape york and tassie from the off cuts you have from shaping the rest of the coast! Give it a whirl, i'd love to see the pics.

If you decide it is all too hard, you could make a flag cake with the aussie flag on it......