That's it Passive one, not only do we have natural population growth but we also have lots of immigrants coming over.
South Africans and Pom's are coming over in droves and they have money, poms in particular after selling their modest properties in England by Mcmansions here and have money to spare.
Then there is Asia and China, the amount of wealth there is beyond imagination and Australia is seen as one of the safest and best places to live in the world.