its not as easy as them just throwing 1b people at us lol

Why would they? And how would they hope to maintain supply lines across thousands of kilometres of open ocean and then hostile terrain??? Unlike France, the US or the UK, China is not a current or former colonial superpower with military bases all over the planet, so almost all their aircraft and ships would have to come all the way over from China for every trip. The fuel costs of a sustained invasion and subsequent occupation would be staggering. What would they want that could possibly be worth that kind of attrition hell that they couldn't get for so much less trouble by just trading for it?
Yeah okay, if it happened, and if they were prepared to throw as many resources into the meat grinder as it'd take, China would probably 'win,' but that doesn't mean that we've got any reason to believe this has any chance of actually happenning because it's so pointlessly stupid.

In a vacuum yes, but we do not live in one. The US doesn't even let China invade Taiwan. That's a place it has a claim to, tenuous as it may be. Australian defence has always relied on stable international relations and allied support. You're delusional if you think cutting off China from Australian resource markets will make us the biggest fish in the pond.