US officials have warned there is a chronic shortage of spare parts for international buyers of the Joint Strike Fighter.

Key points: F-35As are not mission-capable 48 per cent of the time

F-35As are not mission-capable 48 per cent of the time The network to distribute spare parts for the F-35 to international partners has not been established

The network to distribute spare parts for the F-35 to international partners has not been established US official says the fighters "probably won't" meet a September benchmark for capability

Australia is one of eight international partners in the Joint Strike Fighter program and the Federal Government has committed to buying at least 72 of the F-35 fighters at a cost of approximately $124 million each, according to former defence minister Christopher Pyne.

"The bottom line is: far too often the F-35 is on the tarmac because there aren't enough spare parts," said Diana Maurer from the US Government Accountability Office (GAO) in her first interview with Australian media.

In April, Ms Maurer delivered the findings from an investigation she led into the Joint Strike Fighter program.

Her report found the F-35As were not "mission-capable" 48 per cent of the time, meaning the aircraft were not able to fly a single mission.

The US Department of Defence, which owns all spare parts, has spent billions of dollars on them but does not know how many it has or where they are, according to Ms Maurer.

Each of the F-35A Joint Strike Fighters costs $124 million. ( ABC Newcastle: Ben Millington )

The plan to create a network of repair facilities for the aircraft is also running eight years behind schedule and will not be fully completed until 2024, after the F-35 enters service.

The network to distribute parts to international partners like Australia has not been established.

Some 1,000 F-35s are expected to enter service around the world in the next two or three years, and Ms Maurer said it was essential the infrastructure would be ready to support the aircraft.

"It's really important for the F-35 program to address all the numerous problems we found with the global support network that is supposed to underline the program," she said.

F-35 could also miss basic performance benchmark

Last year, then-defence secretary James Mattis ordered the F-35 must be capable of flying at least one tasked mission at least 80 per cent of the time by the end of September this year.

"I think that's a stretch, in my opinion, I think that probably won't happen," said Mike Sullivan, Director of Defence Weapon System Acquisitions at the GAO.

Australia already has four F-35s out of a total commitment of 72. ( ABC News: Andrew Altree-Williams )

Mr Sullivan said the F-35 has suffered as a result of new capabilities being jammed into the fighter at a late stage of development, resulting in redesigns and supply chain changes.

"The manufacturing floor got very chaotic and so you wind up with aircraft coming off the line that are just not very up to snuff when it comes to reliability and parts and things like that," he said.

Australian expert warns against public criticism

International partners, like Australia, have committed billions to the F-35 and are now in a position where they have to make it work.

"From an Australian perspective, we need to make sure the sustainment of that platform is effective," said Dr Malcolm Davis, from the Australia Strategic Policy Institute, a government-funded think tank.

"We have to work with the Americans to get these problems fixed as soon as possible."

Even some former critics of Australia's involvement in the Joint Strike Fighter program agree.

"We can't turn back," said Keith Joiner, an aeronautical engineer and former Director-General of Test and Evaluation for the Australian Defence Force.

"Putting deficiencies into the public domain isn't helpful, this is now a matter of national security."

Mr Joiner had previously called for Australia to delay the purchase of F-35s until Australian authorities had a clearer idea about their capability and suitability.

"This is what our pilots will have to use in a war, so we don't do anyone any help by discussing deficiencies," he said.

Delays could shorten the life of the fighter

The F-35 is still the most advanced fighter jet in the air today, according to Dr Davis.

"Having said that, it has taken a long time to develop this aircraft, we're talking 20-plus years," he said.

Former prime minister Tony Abbott gives a thumbs-up from the cockpit of an F-35 Joint Strike Fighter. ( AAP: Alan Porritt )

The delays in the F-35 becoming fully operationally capable could mean they spend less time at the top of the food chain, according to Dr Davis.

"Our adversaries, the Chinese and the Russians, will have had all that time to understand and study the F-35 and devise countermeasures," he said.

"I don't think its effectiveness will remain quite as long as Defence are hoping it will be."

Dr Davis said he believes this will lead to, "ultimately, replacing the F-35 by the late 2030s".

He said Australia should already be looking at developing replacements for the fighter.

They'll start being operational in 2020

In a statement, the Australian Defence Department said the mission capability figures from the GAO included older versions of the F-35A with poorer mission capability rates than those ordered by Australia.

"Defence is aware the US Secretary of Defence set a requirement for 80 per cent mission capability by 30 Sep 2019," read the statement. "Although this US requirement will benefit all F-35 users, Australia's next significant milestone is Initial Operational Capability in December 2020."

The first four of Australia's F-35s have already been delivered to Williamtown airbase near Newcastle.

The total order of 72 fighters will cost $17 billion.