Yeah, where else would you invade from though? It's an island with no neighbours but North, the only land close to it is north, major threats like China are north. The reef would make it hard to go east by sea too I think.

I doubt anyone would bother invading Australia anyway, it's just not worth the effort. Small population probably going to make a minor impact in wars with larger nations and fairly good allies to the north through all those islands you'd have to sail past.