Australia's population is set to double to more than 46 million by 2075, experts forecast.

The Australian Bureau of Statistics (ABS) on Tuesday released its predictions for coming decades – based on fertility, life expectancy and migration – including that population growth is likely to be greatest in capital cities.

"Melbourne and Sydney should be neck and neck by 2053, with 7.9 million people each," the head of demography at the ABS, Bjorn Jarvis, said in a statement.

The head count in Perth will total 3 million by 2028, when it will overtake Brisbane.

A decade later the ACT's population will surpass Tasmania’s.

Jarvis said the projections were based on "medium growth" and that the nation could reach a 46-million population as soon as 2058.

Growth is expected to continue in all states and territories except Tasmania where it is thought the population will plateau by 2040 and fall from 2047.

Increasing life expectancy will lift Australia's median age to 40 years by 2040, up by three years on 2012 figures.

Over the same period, the number of people aged over 85 is expected to almost triple to 1.2 million, while the number aged over 65 will double to 6.8 million.