Defence Minister Richard Marles will unveil the government’s plan to expand the current fleet of 11 combat ships on Tuesday as the defence force pivots to project its firepower deeper into the Pacific.

It’s expected that the navy’s fleet of warships will be boosted to more than 20 under the shake-up, but there has been wide  speculation that the frigates to be built at Osborne in Adelaide will be cut from the nine announced by the former Coalition government, to six.

Marles has previously criticised the former government for underfunding the frigate program, saying the actual cost of the nine vessels was closer to $65 billion, against the $45 billion flagged.

The exact number of vessels to be announced today has not been confirmed, but Marles offered some clues in the public version of the foreword to the review, to be released later today.

The surface fleet review was sparked by a broader inquiry into the Australian Defence Force’s readiness to tackle future challenges.

It recommended Australia boost the navy’s lethal capacity.

The fleet will be “twice as large as planned when we came to government … and with more of these new surface combatants in the water and operational sooner”, Marles said in the foreward, according to excerpts seen by AAP.

The minister also agreed to commit to continuous naval shipbuilding to get boats off the production line.

The review of the surface fleet is also expected to make 18 recommendations after considering the fleet’s makeup, shipbuilding schedule, cost and workforce issues.

It is also expected to find current plans for the navy’s fleet aren’t enough given the risk Australia faces on top of underlying cost pressures.

The current operational fleet is believed to be the oldest in the navy’s history.

Every review in the past 50 years has said the navy needs 16 to 20 ships, national security expert and former navy commander Jennifer Parker said on Monday.

“The challenge for the Royal Australian Navy has always been that it doesn’t have enough ships,” she said.

The navy is also struggling with staff shortages and is more than 880 personnel – or 6.5 per cent – below strength.

Given the navy’s crewing challenges, having a larger number of smaller vessels would allow it to operate in more places off the coast, Parker said.

The challenge would be to get adequately armed warships before the ageing ANZAC-class frigates come out of the water and the first of the “underarmed” Hunters come into service in the early 2030s, Strategic Analysis Australia director Michael Shoebridge said.

“But the most well-armed design at the best price is meaningless if it can’t be built and delivered into naval service this decade,” he said.

Any cut to the frigate program would result in the cost for each frigate going up, opposition defence spokesman Andrew Hastie said.

“The question has to be for the government, what can we fix by 2026? Because the strategic situation is deteriorating,” he said.

Premier Peter Malinauskas said last week that the priority was not the number of frigates to be built, but that contracts be signed quickly with BAE and that the federal government commit to a continuous shipbuilding program for South Australia “forevermore”.

– with AAP