The Royal Australian Navy expects its new Evolved Cape Class patrol boats could now be delayed by up to nine months, while an extra $44 million will need to be spent keeping its old fleet in the water, after poor-quality aluminium was imported from China.

In March the ABC revealed Australian shipbuilder Austal had detected deficiencies in the material, believed to have been sourced from Wuhan.

At the time Defence said it expected "the scheduled launch dates of all six boats to be delayed by between four and 16 weeks".

Now an Auditor-General's report has revealed the delays have been significantly upgraded to between six and nine months, partly because Austal is also struggling to recruit skilled labour.

"The ANAO's [Australian National Audit Office] comparison of the date ranges provided by Austal against contracted dates indicates that delays of between six and nine months are anticipated for all six boats' acceptance milestones," it said.

According to the ANAO, Austal advised Defence in June of "further schedule delays due to production workforce issues" in the $350 million project.

For the first time Defence has also publicly quantified how much the delays to the Evolved Cape Class patrol boats will cost in terms of keeping older Armidale Class boats in the water.

The ANAO said that in July this year Defence estimated delays would cost an extra $43.9 million.

"This has resulted in the planned extension of service of the Armidale class and a reduced in-service period for the evolved Cape class, demonstrating the consequential effect of project schedule delays to ADF capability and the Australian Government's naval shipbuilding strategy."

Shadow Assistant Defence Minister Pat Conroy criticised the government's handling of the project.

"This, at a time when our nation's security is of critical importance," he said.

"This is an incompetent, wasteful government that cannot deliver Defence projects on time and on budget.

"As always, taxpayers are forced to foot the bill for their stuff-ups, and our Defence personnel are left without the capabilities they need, when they need it."

West Australian-based shipbuilder Austal was awarded the contract to build six of the 58-metre Cape Class vessels to replace the Navy's ageing Armidale Class fleet in May last year.

A month after the ABC revealed problems associated with the imported aluminium, Austal ended its joint venture with a Chinese shipbuilder called Aulong Shipbuilding.

Defence Minister Peter Dutton has been contacted for comment.