Why would Australia be a civilization? Not a particularly old civilization, imo. The civilization started bascially a few hundred years ago when the English came to the island and used it as a prison camp. If a civilization at all, they should start in the industrial era because they didn´t exist earlier. The key up until today has been that the civilizations in this game series has been either ancient or one of the "colonizing"/war time civilizations (read American, English, France). Australia doesn´t fit any of those criteria.
Sorry for the critical comment, but I think there are so many more civilizations that have earned being part of the game, e.g., Vikings and Persia.