Prime Minister Kevin Rudd's rhetoric on the issue, too, has changed. Last year, as Treasury began to increase its predictions about our future size, he branded himself a big-Australia man. ''I think it's good for us, it's good for our national security long-term, it's good in terms of what we can sustain as a nation,'' he said. Then came Treasury's Intergenerational Report this month and its prediction that Australia would swell from about 22 million people to 35.9 million people in 2050. Rudd was no longer so sure a large population was a good thing, so he instead sat on the fence as the opposition increased a get-tough-on-asylum-seekers theme. Despite changing rhetoric, the reality is our leaders remain hopelessly addicted to population growth. It is a drug they are unlikely to kick any time soon. Size, for some misguided reason, has long been equated with importance. But there is another, more intrinsic reason for the addiction. Population growth is one of the simplest ways for a government to boost economic growth, which is in turn regarded as a key measure of political success. More people means more houses, more cars, more food consumed and more petrol burnt. All this is dutifully recorded by the Bureau of Statistics as an increase in Australia's gross domestic product, which is in turn associated with prosperity. An apparently circular need to feed economic growth with population growth represents a significant flaw in our political system.

First, economic growth for its own sake is not necessarily a good thing. Rebuild a town flattened by bushfires and it's recorded as economic growth. Yet can you really argue that the town's population, or society as a whole, is better off? Second, even if you assume GDP is a good measure of progress, the focus should not be on GDP growth per se, but GDP growth per person. The economic pie may be expanding, but if the number of people sharing it is growing at an even greater rate, then everyone gets a smaller slice. This is exactly what has happened in Australia. We may have been one of the few economies in the developed world to have grown over the past year (just ask Treasurer Wayne Swan). But because the population grew faster, GDP per person slipped by about 1.7 per cent over the year to September 2009. But don't expect to hear Swan talking about that. Labor backbencher Kelvin Thomson, who is arguing for a cut to Australia's skilled migration intake, says there are many costs of population growth not factored into economic growth measures, including environmental degradation, loss of urban amenity, and congestion. ''I believe very strongly that if we add another million, or 2 million, or 3 million people to Melbourne over the course of the next few decades, that will be a poorer city than the one that I have had the privilege to live in,'' Thomson says.

There are other issues, too. About 64 per cent of Australia's recent population growth has been due to migration. As opposition immigration spokesman Scott Morrison points out, the states have little influence over migration levels, despite being responsible for many areas that are affected, including planning, infrastructure and environment. ''Growth is good only if it is managed well,'' Morrison says. ''If it is not managed well it can be very counterproductive.'' The prediction that the population will hit 36 million by 2050 may even be too conservative. The Intergenerational Report assumes an annual net overseas migration for the next 40 years of 180,000. The intake last financial year was about 285,000. As Morrison says, it would be intriguing to know when the government intends to cut the intake to bring about this average figure of 180,000. None of this is to say that immigration is a bad thing. What we do need is to have a sensible debate about how big we want to get. Once we decide this, our politicians will need to make some brave decisions, including acknowledging that economic growth isn't always worth pursing for the sake of it. After all, as former treasurer Peter Costello liked to say, demography is destiny.

Josh Gordon is The Sunday Age national political reporter.