You couldn't have said it better.
While the northern hemisphere is being devoured by radiation, and us Aussie's are just waiting for the Fallout, which would be considerably depleted in mass by the time it reached us, we could just fire off our own nukes at random countries: France, Vietnam, USA, Canada, Germany, Turkey, Austria etc. We would be the last of the civil society as we know it
Yeah, gotta watch out for those silos disguised as barbies...
As for power armor...I hear you can make quite a lot of useful things with empty Foster's cans...
(JUST KIDDING!)
Seriously though, as to vaults and whatnot. Australia has something much of the US (except the plains states) don't have. Millions of square acres of hard-pack flatland, perfect for excavation. If that isn't enough, Australia has some very impressive rocky terrain just perfect for underground installations.
Furthermore, monetary considerations aside, nationalizing various mining companies would provide the government with cheap, almost unlimited labor fully equipped with the best mining machinery the world has to offer, not to mention a ready supply of materials for maintaining and repairing those mining machines (copper, bauxite, iron, diamond dust, gold, uranium, etc). Workers would most likely be given 'first crack' at lotteries for a certain number of vault slots (rebuilding takes more than eggheads and soldiers, after all), given extra rations (enforced rationing would have begun during the resource wars, as well as government stockpiling), and given as many hot air promises as possible.
Australia being situated as it is, would also become a port of call for special forces, scientists and paranoid wealthy folk around the globe who would probably flock there both for it's seeming (even if false) remoteness from the troubles of the world, and it's easy access to excavation and construction resources.
Vaults don't have to have 6" thick steel walls. In the 1950's, fallout and bomb shelters, based on WW2 designs, were all the rage. Companies were building backyard shelters for as little as 1,500 to 2,500 US dollars, usually made by digging an oversized storm cellar and lining it with rebar reinforced concrete ranging from inches thick in cheap 'vaults' to several feet thick in more pricey units.
This 'backyard industry' would probably take off wildly in most countries and Australia, again with its prodigious mining industry, would probably be in the forefront.
I forsee that the communists would probably pop a few nukes at Australia, mainly centered around the capitol, but because they would intend to come and pillage the country later, would spare much of it...people included...after all, dead men don't make good slaves, and China would definately institute slavery to support their efforts to [censored] the nation of it's resources as quickly as possible.
Furthermore, I disagree vehemently with anyone who says the U.S. would say 'screw it' and nuke Australia just to keep it out of the hands of the communists.
So there you have it...a country filled with 'mouse holes', waiting for the fallout to pass and nuclear winter to recede.
Coming out of their holes and vaults, those that survived would be greeted by a land harsher than they remembered..but considering the topography of much of Australia...not by much.
Plantlife would be mostly gone, but topsoil, though irradiated and mixed with fallout would still be there, hardy species that dropped their seeds would begin to spring back, though in mutated form, and indigenous animal life would mutate and adapt to the environment, being naturally more hardy than that found in the US.
I imagine that by 100 years after Armageddon in the FO universe, Australia would have an extensive series of towns and villages in 'outback' style.