Why would Australia even need nuclear weapons?? It barely has an army and our main form of defence is alliances with other countries. Also, who the hell would bother attacking Australia?! It would be a waste of time and money. It's a huge area, geographically and it's population is low. Maybe smaller scale bombings or something, but I would think even those would be executed by Australian residents, not foreign countries trying invade us or something. If you were going to terrorise a country with intent to cause maximum economic destruction and loss of lives (and spend all that time and money developing these weapons) - Australia would not be on top of your hit list, by far. USA, UK, some other European or major Asian countries, maybe.
Well thats my opinion, anyway.