Revealed: Defence Force Boeing 757 fleet unavailable for 156 days since 2019

The New Zealand Defence Force's (NZDF) flagship aircraft used for VIP transport, peacekeeping and disaster relief aren't always available for use.

Chief of Airforce Andrew Clark says "anything can go wrong on an aeroplane at some stage".

Documents obtained by Newshub show the Boeing 757 fleet has been unavailable for 156 days since 2019.

Eighteen of the 26 times weren't planned and there were two occasions when neither plane was available for over a month.

Each of the five Hercules aircraft has all been out of service five times for a total of twelve days. All up it's cost $126 million since 2016.

The NZDF says it excludes associated personnel costs, depreciation, capital charges and/or capital expenditure associated with deep-level maintenance and major overhauls.

"It's always possible you get aircraft going unserviceable while they're in the air and that's managed by the crew as it occurs. More commonly there's an inspection post-flight or pre-flight," Clark said.

The Government's spending $1.5b to replace the five Hercules aircraft in 2024, the 757s won't be upgraded until 2028.

There have been a raft of high-profile breakdowns involving the two 757s which carry personnel and equipment, but also provide VIP transport.

Prime Minister Jacinda Ardern had to deal with a week of breakdowns in 2019, in the same year a second plane had to be sent to pick up former Deputy Prime Minister Winston Peters when one of them broke down in Vanuatu.

Former Prime Minister John Key had to be saved when a 757 broke down twice in 2016, stranding him and a delegation of nearly 100 in Australia en route to India.

The Defence Capability Plan forecasts the two aircraft will be replaced post-2028. Intelligence expert Paul Buchanan admits the planes may not be fit for purpose.

There is concern the capability could be a strategic risk for New Zealand.

"We may find ourselves without the ability to move somewhere quickly on our own and the more we ask our partners to do what we should do ourselves the more they'll question our commitment to our security relationships," Buchanan said.

Pressure is mounting on the Government to act promptly.

"If it's becoming inevitable they're spending more time on the ground than in the air. Then the Government does need to step in and provide some kind of solution," National's Defence spokesperson Chris Penk said.

The Defence Minister wasn't available for an interview, but the NZDF says the 757 upgrade won't be brought forward. Assurances are being made they are safe to see out several more years of service.