It's based on outdated economic theories, along the lines of "more people means more spending, therefore economic growth". It's partly why housing prices and the cost of consumer goods in Australia are so high (by global comparison). A stable or declining population would mean cheaper prices for the average worker, but less profits for business. (Now, guess which one the government works for)?
And the types of migrants also plays a big role. Although I'm broadly generalizing, and mean no offense to anyone, if the balance stays with Chinese and Indians of some means, who value hard work and education, then that feeds nicely into the above outdated economic theories. If it's people from other parts of the world who perhaps don't value work and education as much (ahem), then even the outdated theories will fall over. I'm not convinced our government has figured this out yet.
