Defence Minister Richard Marles said that from July, looser eligibility criteria would allow "permanent residents who have been living in Australia for 12 months" to serve.

Citizens from Britain, Canada, New Zealand and the United States are being favoured, he added.

Australia has a coastline that would stretch one-and-a-bit times around the Earth, but a population of just 26 million.

Canberra has surged defence spending in recent years, buying fleets of submarines, jets and scores of fighting vehicles to meet mounting regional tensions.

But it has struggled to find enough pilots, mariners and troops to operate and maintain them.

Experts warn too few Australians don a uniform to meet even current requirements, much less a beefier military of tomorrow.

The Australian Defence Forces can today count on about 90,000 personnel, including reserves, according to the Australian Strategic Policy Institute.

China's military, by contrast, has an estimated two million personnel.

Marles said growing the Australian Defence Force was "essential to meet the nation's security challenges through the next decade and beyond".