Opposition Leader Tony Abbott has rejected the government's population target and says under the coalition an expanded Productivity Commission would calculate a "rational" figure.
Speaking to reporters in Sydney's northwest, Mr Abbott highlighted the area as a "suburb under pressure" which lacked the infrastructure for future growth.
"Australia's largest cities are choking on their traffic and Australia's environment is under pressure everywhere," he said.
"That's why the coalition rejects Mr Rudd's Big Australia population target of 36 million by 2050.
"It's not that we are necessarily against a larger population but we have no confidence whatsoever in the prime minister's competence to deliver the kind of massive surge of infrastructure that would be necessary to ensure that Australia remains a livable country with this kind of a population increase."
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Based on current immigration rates, Mr Abbott said Australia's population would reach 42 million by 2050. "That's almost a 50 per cent increase within 20 years," he said.
"There is no evidence that Australia's governments are building the additional infrastructure needed to sustain a new city the size of Canberra every single year."
Mr Abbott declined to nominate what population forecast figure he deemed appropriate but said that if a coalition government was elected, it would undertake fresh analysis.
"What we need is a rational basis for making decisions about Australia's future population and about Australia's future immigration numbers," Mr Abbott said, adding he would reconstitute the Productivity Commission as the Productivity and Sustainability Commission.
"(We) will task that reconstituted commission with giving the government independent expert advice on the kind of population numbers that are sustainable in the short, medium and long term," he said.
Mr Abbott on Thursday released a policy directions paper, under which the coalition committed itself to putting Australia on a path to deliver economic productivity, environmental sustainability, liveability and border security.
