By Jonathan Mitchell for RNZ

Documents obtained by RNZ showed on one occasion in March, a plane was delayed for 72 hours in Singapore because "NOGO" was displayed in the Master Caution Display Unit.

In May, a plane was delayed for 24 hours in Sydney after a fuel tank was overfilled because a pump was stuck in the on position.

And in June, after landing at the Royal Australian Air Force base in Amberley, a plane was held up for five hours because of a master warning alert and a fire indication - although no fires were found.

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Last month, the prime minister and her staff had to return to New Zealand on a commercial flight after an Air Force 757 broke down in Melbourne.

The Hercules fleet dates back to the 1960s and is due for replacement by 2023.

David Capie, who's with Victoria University's Centre for Strategic Studies, said the breakdowns were a reflection of the age of the fleet and the replacements couldn't come soon enough.

The Hercules are old and need to be replaced urgently, David Capie says. Photo / Supplied: NZDF
The Hercules are old and need to be replaced urgently, David Capie says. Photo / Supplied: NZDF

"They're just old ... frankly it is amazing that they've managed to keep them still flying - but it really just underscores the urgency of getting these new aircraft into the fleet and flying," he said.

The documents showed $20.2 million was spent on repairs and maintenance of the Hercules fleet in the last financial year.

In the past and current financial years, about $32m was earmarked for the Orion fleet to keep it fit for purpose, before those planes were being replaced with P8 Poseidons.

Mr Capie said the replacement planes weren't expected to be as much of a burden.

"There will be different costs obviously - but in terms of that kind of maintenance and the amount of time they're out of service - that will certainly be much lower," he said.

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- RNZ