Loading Demographer Liz Allen, from the Australian National University, said it demonstrated population growth was "not a problem", and that "politicians will have to stop using population as a scapegoat for policy and funding failures". The ABS presents three models: high-growth, medium-growth and low-growth projections, based on different factors such as the fertility rate, life expectancy and the migration rate. In the medium-growth model, Australia's population would hit 30 million in 2030-31, 35 million in 2043-44 and 40 million in 2058-59. By the year 2066, it would be 42 million. In the highest growth scenario, the population by 2066 would be as high as 50 million. The bureau also expects urbanisation to continue across the country, with every capital city to grow faster than its state or territory. It projects Brisbane will be a city of about 3 million people by 2027, and will for the first time contain more than half the population of Queensland, Australia's most decentralised state.

Victoria will see the fastest growth of any state. In the medium-growth scenario it would rise from 6.3 million to reach 12 million people by 2066, or 14.5 million in the highest-growth scenario. In the fastest growth model, Melbourne would overtake Sydney as Australia's biggest city as soon as 2031. In the medium-growth scenario that would happen a few years later, in 2037. The bureau made a number of different projections for NSW, which is expected to grow slightly slower than the rest of the country. In one scenario, in which net interstate migration away from NSW was lower, the population of NSW would grow from 7.9 million now to 15.5 million by 2066, and the population of Sydney would increase to nearly 12 million. But in another scenario, in which the interstate exodus from NSW increased, the population would reach just 11 million by 2066 and Sydney only 8 million.

The bureau's more conservative projections assume a life expectancy of 83 years for men and 86 years for women. The medium-growth model assumes a fertility rate of 1.8 babies per woman and net overseas migration of 225,000 people annually. Dr Allen said the assumed fertility rate was "a bit ambitious" and that Australia may not actually reach the medium-growth projections modelled by the ABS. Australia hit the 25 million milestone in August and the ABS population clock puts the current population at just over 25,143,000. That grew from 20 million in October 2004, and the bureau expects to hit the 30 million milestone between 2029 and 2033. "The projected time for the nation to grow by 5 million people on current indications will be similar, if not a little shorter, than the 14 years it took to grow from 20 million to 25 million," said Anthony Grubb, director of demography at the ABS. Australia's population will also continue to age, and the average age could hit 40 by the year 2066, according to the bureau's projections.

Loading The number of Australians over 85 is expected to double by the year 2042, while those over 65 will almost double from 3.8 million to as many as 6.7 million. The retirement age - at which people qualify for the age pension - will increase to 67 by the year 2023, although Prime Minister Scott Morrison scrapped plans to lift it to 70. "We're an ageing population that relies on migration to offset the potential adverse consequences of that ageing population," Dr Allen said. "Migration is what's keeping our economy afloat." Mr Morrison this week flagged his intention to put the brakes on migration, which, if permanent, would affect population projections.