The cost of Australia's largest defence projects has soared by billions of dollars, and they are now facing a cumulative delay of 57 years.

Key points: An auditor-general report on 26 major defence projects finds cost and time blowouts

An auditor-general report on 26 major defence projects finds cost and time blowouts It found costs have risen $24 billion since the projects were first announced

It found costs have risen $24 billion since the projects were first announced Delays on the projects now add up to a total of 57 years

An auditor-general's report analysing 26 major defence projects has found the projects' budgets have risen more than $24 billion since they were first announced, and will now cost taxpayers $64 billion.

Among the projects examined by the Australian National Audit Office (ANAO) were Joint Strike Fighters, Air Warfare Destroyers, P-8A Poseidons, MRH90 helicopters, Offshore Patrol Vessels, Growler aircraft and Collins Class submarine reliability and sustainability.

Together they represent half of the total approved budget for the Defence Department's active major and minor capital equipment projects.

In the past year alone, the cost of the 26 projects rose more than $1 billion, according to the ANAO's report.

"Cost management is an ongoing process in Defence's administration of the major projects," the auditor-general found.

Joint Strike Fighters accounted for most of the increase, with the project's cost rising almost $14 billion after the Government ordered another 58 aircraft.

Then-defence minister Marise Payne welcomed the first Growler aircraft to Australia in 2017. ( ABC News: Cameron Best )

Significant delays have also been recorded, with the ANAO finding the total schedule slippage for the selected projects is now at 691 months, or 57 years.

That represents a 27 per cent increase since second-pass approval, or scoping phase, for the projects, with average delays at 2.7 years.

Defence Minister Linda Reynolds put the increase in cost down to the state of the Australian dollar and defended the spending of the Morrison Government.

"The Major Projects Report released by ANAO confirms that within the 2018-19 review period, Defence's 26 major projects continued to deliver strongly against measures of scope and cost.

"The reported increase in the cost of delivering these projects over the past year was purely a consequence of the fluctuating rate of the Australian dollar and price indexation during this period."

Crossbench senator Rex Patrick said Defence often spent hundreds of millions of taxpayers' dollars getting a project to second-pass, where it could be presented to Cabinet with a known scope, budget, schedule and risk profile.

"The ANAO report, which references cost and schedule overruns to second-pass, reveals that Defence is incompetent in its original assessments, and that there is a need for the Defence Minister to intervene and make changes," he said.

"It's not just the schedule and cost blowouts in the original project that needs to be considered.

"There's the cost to the men and women in the Defence Force who are forced to carry out their tasks with obsolete equipment.

"There's also the financial cost of Defence of keeping old and tired equipment running."