Because it’s bollocks? The figure you are looking for is quarter of an acre, not “a couple”.
http://www.youtube.com/watch?v=lUnvm0qhe2g
And that kind of ignores ALL the resources needed to support those people and the fact that most of Australia (like a large amount of the world) is basically uninhabitable. My mistake. However, 1/4 of an acre is a lot more area than my residential home occupies.
But that’s not the point. If everyone in a 5 bedroom house could fit into one wardrobe, no-one’s saying everyone in the household should live in the wardrobe, but it shows the house is not overpopulated.
So if every human can fit onto Australia with a quarter acre each, leaving every other land mass empty, does that sound like overpopulation? No, it doesn’t.