IT’S the debate we were never allowed to have.

Until relatively recently, Australia’s population grew at a stately pace. There was an influx of European immigration in the mid-1940s, and pause from the mid-1970s, but in the 100 years after Federation in 1901, net overseas migration averaged 70,000 people a year.

Then in the early 2000s, Prime Minister John Howard opened the floodgates. Over the last 12 years, Australia’s annual net overseas migration has tripled from its long-term average to 210,000 people per year.

Our cities are bursting at the seams, roads and services are congested, and house prices are skyrocketing — particularly in Sydney and Melbourne, which attract the lion’s share of new Australians.

Over the last 12 years, Sydney has added 20 per cent to its population, or 800,000 people. Melbourne has added one million people over the same period, or 27 per cent.

According to state government projections, Sydney will add another 1.7 million people over the next 20 years, which works out to 87,000 people a year, or 1650 people per week. Melbourne is forecast to add 97,000 people per year, or around 1870 people per week, for the next 35 years.

“It’s clearly unsustainable,” said Leith van Onselen, chief economist with MacroBusiness. “The problem isn’t that immigration is good or bad, it’s just that the level is far too high for Australia to digest.”

According to Mr van Onselen, dubbed the “Unconventional Economist”, Howard “effectively ran a bait-and-switch policy”.

“He scapegoated the very tiny number of people coming by boat, and at the same time opened the floodgates on people coming by plane,” he said.

“Howard never articulated why he was doing that, he just did it, and unfortunately the following governments, Rudd, Gillard, Abbott and now Turnbull, just followed.”

Mr van Onselen, who is one of the few public commentators calling for a national debate about Australia’s annual migration intake, says there is now “tri-partisan support” between the Liberals, Labor and even the Greens to not discuss the issue.

Behind the scenes, the “growth lobby” of retailers, the banking sector, the property industry and “erroneously named think tanks” all push the “growth-ist agenda”. “Unfortunately there’s not really anybody on the other side,” he said.

Late last year, high-profile entrepreneur Dick Smith came out in support of Pauline Hanson, warning that Australia would be “destroyed” if One Nation’s immigration policies weren’t taken seriously.

Mr Smith had previously spoken out about the need for a “small Australia”, with a population of 26 million rather than 50 million. At current migration levels, Australia’s population will hit 40 million by the year 2060, compared with 33 million if the intake returned to its historical average of 70,000.

“Unfortunately you can’t have a sensible debate,” said Mr van Onselen. “The main problem is the perception of racism. The easiest way to shut down debate is to call someone racist. Our politicians and media won’t mention it because they’re afraid they’ll get associated with Pauline.

“It’s nothing to do with race — it’s an economic and living standards debate. It’s purely a numbers game, that’s all that matters. A body is a body. If you’ve got an extra car on the road, an extra person on the train, it doesn’t matter where they’re from.”

The common public argument used to promote mass immigration, particularly by the likes of the United Nations, is the need to replace an “ageing” population. The behind-the-scenes rationale is to artificially boost economic growth numbers.

Both justifications fail to stand up to scrutiny. According to the Productivity Commission, which has debunked the ageing population myth numerous times over the past 15 years, “changes in migration levels ... make little difference to the age structure of the population in the future, with any effect being temporary”.

“The reason is very simple — immigrants grow old,” said Mr van Onselen. “You can bring in a whole bunch of young people now, it will lower the age temporarily, but in 30 years time those young people are old and you have to repeat the same trick all over again. Really it’s just a Ponzi scheme.”

Which ties into the second justification. Japan, with its sluggish headline economic growth and simultaneously ageing and shrinking population, is commonly cited as an example of why mass immigration for population replacement is necessary.

At the same time, Australia’s record run of economic growth, coinciding with record immigration levels, is held up as a positive example. “All other things being equal, if you increase the population by 1.5 per cent a year, you’re going to get 1.5 per cent economic growth,” said Mr van Onselen.

“More inputs in people means more outputs in economic activity. But the problem is, although it makes the overall growth figures look good, it doesn’t actually help you on a per capita basis, which is what drives living standards.”

In fact, despite Australia’s population surging 21.5 per cent since 2003, compared with the OECD average of 8.5 per cent, Australia’s GDP per capita change has just barely outpaced the OECD — 16 per cent versus 15 per cent, despite going through the biggest mining boom in our history.

“We’re effectively spinning our tyres importing all these people, wearing out our infrastructure, making housing more expensive and degrading the environment for absolutely zero gain, in the material sense,” he said.

“The immigration program used to be a supplement to the economy, now it’s seen as a driver. Governments are using it as a lever to stop Australia going into recession. The tail is wagging the dog.”

Japan, meanwhile, has grown its GDP per capita by 11 per cent since 2003. “Japan’s unemployment rate is nearly half of ours,” said Mr van Onselen. “It’s hardly a terrible situation they’re in. They’ve got good growth at a per capita level and basically anyone who wants a job can get a job.”

According to the UN’s Population Division of the Department of Economic and Social Affairs, “replacement migration” is the “solution to declining and ageing populations”.

“Population decline is inevitable in the absence of replacement migration,” the UN said in a recent press release. “Fertility may rebound in the coming decades, but few believe that it will recover sufficiently in most countries to reach replacement level in the foreseeable future.”

Mr van Onselen described it as “ridiculous”. “The UN pushes a sort of open borders, globalist agenda,” he said. “It is a myth. We just need a national debate. There’s no strategy, it’s all just ad hoc. How big do we want Australia to become? How are we going to accommodate people? Is this what people want?”

Writing in The Australian, economist Judith Sloan pointed out that in 2011, Malcolm Turnbull made the “astonishing claim” that “anyone who thinks that it’s smart to cut immigration is sentencing Australia to poverty”.

“It is important that we have a measured and informed debate about our immigration policies, in terms of both numbers and the integrity of the visa categories,” she wrote.

“Are people really happy that Australia’s population will exceed 40 million in 2060? Are we really testing for skill when we set the visa categories? Has the migration program simply become a way of allowing universities to charge very high fees to international students on the understanding that the graduates can attain permanent residence?

“These are the questions we should not be afraid to pose and politicians should not be afraid to answer.”

Greens immigration spokesman Nick McKim told news.com.au: “The Greens believe in a broad and non-discriminatory immigration policy. In particular, we believe that Australia’s humanitarian intake should be increased to 50,000 people per year.

“Australians are a friendly and welcoming people and we have long and proud history of multiculturalism, which has added so much to the fabric of our country.

“There will always be debates about immigration, and it is disappointing to see so many commentators and politicians resorting to xenophobia and racism.”

Immigration Minister Peter Dutton and Labor immigration spokesman Shayne Neumann did not respond to requests for comment.

frank.chung@news.com.au