Knowing that Australia is one of those British Empire nations that does not trust it subjects, I would not worry about the citizens having guns and fighting tooth and nail. So all an invader would have to worry about is the armed forces, such as they are. First an invader would have to take out the Australian Navy, a whole 60 ships with no aircraft carriers. The only ships that I would worry about are the six diesel-electric subs. Everything else is a surface ship and would be relatively easy to take out with Harpoons or Exocets. Second, the Air Force. They have 20 or so Vietnam era F111s that could be used for long range recon and as a medium bomber. They are due to retire in 2010 so if I was worried about them I would wait two years. They have about 110 F18 fighters. Those are not air superiority fighters like the F15 or F14 so if I got a plane specifically designed for air superiority, I would not worry about them either. Third, having taken out the Navy and the Air Force, I would have air superiority. All that would be left to deal with is the Army. 45,000 total, including Reservists. Hardly a force to worry about. When you consider how big Australia is and how poor the roads are, it would be unlikely they could mass forces if one area was invaded. If several invasion points were used, they could maybe meet an invasion with 10,000 at most. Lacking air cover, they could easily be taken out with a neutron bomb or chemical attack. The ONLY thing Australia has going for them is distance from their enemies, large size of their nation and the ability to burn their northern cities and to flee to the other side of the desert. That was their plan in WW II until MacArthur told them it was a bad plan. Now that you have an idea of how easy it would be, why do it? Just exactly does Australia have that would make someone spend the time and effort to invade? They have no major oil fields, they are not a "bread basket" of food and in fact they are having drought problems, they are not an industrial power house, etc. It would cost more to invade than what benefit you could get doing it. In fact, it is not worth the trouble as you would probably have the entire British Empire coming after you if you tried it. While the Armed forces of Australia are not much, you add in all that of the Commonwealth and you might have a problem. Specially since Britain is a nuclear power.