when we were under threat of invasion, most of our forces were in Europe fighting for bloody Britain.
In Africa actually, and again, the Australian troops at that time in the Middle East were volunteers. They joined up to fight for Britain (and Empire), and that's where the fight was. When the Pacific war began, most (but not all) came back to the Pacific theater to defend Australia because that's where that fight was. Australians (apart from the RAAF) did not have a great deal of participation in the European theater.