If you suggest that Australia was untouched by Nuclear War then its very simple.
You cannot say we'd collapse due to the amount of reliance we have on imports and exports for two reasons.
1. Fallout is a different universe where countries are a lot more politically isolated from one another, we would of had to have adapted.
2. Imports and exports in Australia falls along the lines of crap that is often not essential for survival. In Australia we have raw materials, food and water, the main things we need to survive as a nation.
I also think that its worth noting that in no way would Australia allow anyone to come to its shores. They would block it off from everyone except New Zealand. The Survival and prosperity of the Australian people would be the Prime Minister's first concern, with no UN in place there's no rule telling us to do anything. They'd either tell them to move along or we'd sink their ships.
I highly doubt this has all happened to Australia, I think we were hit in the Fallout Universe and we still survived.
If we were so unaffected, we'd be the most powerful nation in the Fallout Universe, and there would probably be Australian SASR doing reconnaissance in America to see if anyone survived. Then they'd see the Enclave and BoS and probably think to themselves 'Screw this, yanks are killin' each other. Lets go check up on England'.