"Bigger Australia Certain"

A report by the Centre for Independent Studies examined 36 different scenarios and found that the population of Australia would increase under all of them except when migration was cancelled, births forbidden, and people not allowed to grow older. Not a surprising conclusion. Did their scenarios not include plague, nuclear war, of armageddon?
The report concludes that "even if" migration were cut in half, to 70,000 a year, the population will still reach 29,000,000 by 2050. The study seems to say "well, that's it mate, just put up with overcrowding."
But my take on that report is that it seems to make cutting migration an absolute imperative! If home grown population increase is so huge, and our infrastructure so stretched, then why are we considering letting anyone else in from outside?
There's an interesting article in the Economist reporting that the number of people in each household in America is increasing, as the economy encourages families to stay together longer. In Sydney, with its high house prices and the large houses that we are building, this trend is likely to happen here, so fewer houses would be needed to house whatever population we end up with.