Peter Dutton said he had planned to buy two Virginia-class submarines from the United States by 2030 and build another eight to bring the total fleet strength to 10.

The project is the centrepiece of Australian efforts to toughen its defences in the face of a more belligerent China under President Xi Jinping.

The choice of contractor -- Britain or the US -- will have a significant economic impact and would closely immesh the Australian navy with that of the winning nation.

Dutton, who leads the opposition after his conservative coalition lost May elections, said it became "obvious" to him as defence minister that the US submarines were a better choice.

Dutton's disclosure in an article for The Australian appeared to be aimed at pushing the new government into following his plan.

Australia's former government had agreed to acquire either US or British nuclear-powered but conventionally armed submarines as part of a three-way defence alliance sealed last September, known as AUKUS.

Australia is still conducting an 18-month study of its nuclear-powered submarine options as part of the AUKUS deal.

The cost of building even a smaller fleet of eight -- Australia's original plan -- has been estimated at Aus$70 billion (US$50 billion) at an "absolute minimum" and before accounting for inflation, by the Australian Strategic Policy Institute think-tank.

- 'Very unusual' -

"Clearly these were confidential discussions that he had with the Americans which he was not prepared to disclose while he was in office, and yet he did so afterwards," said Sam Roggeveen, director of the Lowy Institute's international security programme.

"It's very unusual."

The US submarines were capable of launching missiles vertically and were based on a "mature design", Dutton wrote.

The British Astute-class option, however, involved a new design with "inevitable" cost blowouts and design faults, he said.

Dutton said nuclear-powered submarines are needed to compete with China because they are stealthier than Australia's diesel-electric vessels, with no need to surface to recharge batteries.

To avoid a gap in replacing Australia's ageing fleet of six Collins-class submarines, Dutton said he had planned to directly buy two of the US submarines "this decade".

The purchase would avoid having to wait until 2038 for the first US-designed submarines to be built in Australia, as the government had promised, he said.

Under the plan, another eight of the US submarines would be built in South Australia, however, bringing the total fleet strength to 10, Dutton said.

To "honour and respect" the losing British side, Dutton said he would have ordered more Hunter-class frigates or other defence material from Britain.

Dutton said he feared the new government was "on the cusp of making a very dangerous decision" such as building a new class of diesel-electric submarines.

Australia's defence ministry has been asked to comment.