> I'm not convinced that if there were 300m Australians, that they'd still all live in those 5 cities (with every city being 10x bigger). I think there'd be more of them.

I don't think so either, but because of the climate and geography, I also don't think there'd be 10x more cities, similar populations, I think you might end up with 2-3x more, really, at most.