The Defence Force says it's looking to replace the 757 plane used to transport the Prime Minister and cargo, but not for several years.

RNZAF Boeing 757-2K2. NZDF. (Source: Supplied)

Air Vice-Marshal Tony Davies says the aircraft may be old with many faults, but says it's safe.

"We have a very high safety bar. It's way up here. If we're ever in doubt, we don't take short cuts and fly in a compromised safety position" he said.

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It comes as the 757 broke down on the final leg of the Prime Minister's recent trip to the US.

Jacinda Ardern was flying home commercially anyway because of the multiple stops needed to refuel, but the rest of the delegation got stuck in the US.

The Defence Force admits there will be more faults as the 2028 replacement date nears. New Zealand has had the planes for 30 years and they were second-hand when they were obtained.

When New Zealand bought the 757 many other countries used them too, so it was easy to maintain. Now that's becoming harder.

The Defence Force said there were no faults recorded when the Prime Minister went to Singapore and Japan but says that's "not normal".