I really don't think fighting would solve anything. Ok, so I pull my $150 Katana out, but what to do when it breaks? Oh. I'm so dead.
No, no, better to try and not piss the invading force off. If I get the chance, I'd hide, John Marsden style (You know, except without the ironic hell analogy and bridge-related guerrilla warfare. I figure foresty area would work, unless the invaders don't care about resources and just torch the fucking place)
But otherwise? If people go out of the way to invade Australia, it sure as hell isn't because we pose a significant military threat. No, they want some of our precious Australium. If I ignore them, they might just shove me into a camp, or something, to mine said Australium.
Of course, when that proved itself dangerous and painful, due to various abuses suffered? I don't know. Everyone has a breaking point. I'm sure that right now, given a chance, I would NOT be able to kill anyone, regardless of present circumstances. After a year of mining in full BDSM gear? Well, you never know.