Australia should consider whether immigration can continue at existing levels as part of a comprehensive population analysis aimed at determining how many people the country can support, the Federal Opposition says.

Concerns about infrastructure, housing and environmental sustainability should be considered when setting the number of people allowed to immigrate each year, Coalition immigration spokesman Scott Morrison said. ''Population policy is a legitimate debate we have to have and it should be free from any suggestion that it's related to race.''

Population policy was a void that needed to be filled, he said. ''It's getting to the point where we can't afford not to [have one]. We can't just keep going as is.''

Treasury modelling released last year forecast the population would increase more than half to 35 million by the middle of the century. The increase will come from migration, more women reaching child-bearing age and higher fertility rates.

Prime Minister Kevin Rudd welcomed the modelling, saying he was in favour of a big Australia. But it prompted criticism from Labor backbencher Kelvin Thomson, who has questioned whether the country can support such a population.