The coalition is backing a federal Labor MP's call for a national debate about Australia's population growth.

Kelvin Thomson, the member for the Victorian seat of Wills, believes Australians are hungry for a debate about how big a population the nation can sustain.

Government projections indicate the population could reach 35 million by 2050, with Melbourne and Sydney each accommodating seven million people and Brisbane five million.

"We are sleepwalking into an environmental disaster," Mr Thomson wrote in The Australian newspaper on Wednesday.

 

There was a need for a debate about population growth and the impact it was likely to have on Australia's carbon footprint, declining housing affordability, traffic congestion, and "overcrowded concrete jungles".

"I detect there is a hunger in the electorate for a debate about this issue," he later told Fairfax Radio Network, adding he had been deluged with emails and letters since first raising the issue.

The opposition agrees, but has stopped short of supporting Mr Thomson's call for a population cap of 26 million.

Nor will it support the MP's suggestion for a reduction in the skilled migrant intake to 25,000 a year.

"We're very happy to have a population policy debate," opposition immigration spokesman Scott Morrison said.

"But we have to make sure that when we're running a migration program we're ensuring that we're bringing people into the country who can really make a contribution."

When asked to nominate an upper limit to Australia's population, Mr Morrison said: "At the moment I don't think we know the answer to that question".

The carrying capacity of the nation's infrastructure and the ability of the environment to sustain a bigger population was not known, he said.

Mr Thomson also wants overseas students to return to their country of origin for at least two years before they can apply for permanent residency in Australia.

He repeated his call for the abolition of the baby bonus and limits on family tax benefits for a third and subsequent children.