Could the Australian military resist an invasion?
The Australian Military is small but quite capable, Australia itself s a very strategically defensible country, and few countries have much force projection capability these days.
Could it resist an invasion from the US? No way.
The UK? Probably not.
France? Probably not.
Korea? (either one) Probably not.
Israel? Probably not.
Pakistan? Probably not.
Any other country? Probably.
Any country below the top 20 or so most powerful (including the ones I listed)? Absolutely.
Realistically, only the US has the logistics capabilities to successfully invade Australia. China, Russia, and India have the strength (and human right's record) to pretty much just brute force it, any of the "probably not" countries could stand a chance under the right leadership, and the maybe's might stand a chance. No other country could do it remotely realistically, barring unseen factors or sheer incompetence on Australia's part (though I left it as probably because you never know). For any country other than the US, it would be a very long and bloody affair.
But Australia is a well liked country, is very close allies with the US and UK, and has a good relationship with China, and with the exception of maybe Iran, no power that could threaten Australia has any interest in getting on the wrong side of any of those three countries, so from a strategic perspective, Australia is untouchable, much like Canada or Switzerland.