Two 500kg external gas tanks were fitted to two Royal New Zealand Air Force helicopters as they flew between two countries for the first time, and crossed a large swathe of ocean.

The NH90 Number Three Squadron helicopters recently made their trip from Cairns, Australia to Papua New Guinea.

They flew 453 nautical miles, or 838.9km over four hours across the Coral Sea Basin to Port Moresby, which marked the NH90s furthest trip over water.

Prior to their flight, the helicopters deployed to Australia for Exercise Talisman Sabre. The multinational military exercise, hosted by Australia and the United States, involves 19 countries and nearly 40,000 personnel, including around 700 New Zealanders.

The helicopters then made their journey to Papua New Guinea to take part in their 50th independence celebrations.

Royal New Zealand Air Force air component commander Air Commodore Andy Scott said the timing of both events was a "fantastic opportunity" for the rotary wing team to take part in both the celebrations and in supporting the transport of critical equipment out into the provinces.

Flight Lieutenant Hayley Vincent,one of the pilots who flew the journey, said two 500kg external gas tanks were fitted, giving the aircraft a "different silhouette".

An extra internal gas tank was also fitted, but the fuel was not required. The Air Force also said the flight was given an international Kiwi call-sign instead of the Warrior call-sign used for national flights.

Before the helicopters left for their journey, a long-range flight was carried out between Base Ohakea to Invercargill.

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“A fuel graph was generated, predicting the amount of fuel burned at various waypoints, taking into account aircraft weights and details, helping us keep an accurate track of fuel burn during the flight, with trends to help with decision making,” said Vincent.

“It showed the expected departure fuel, the expected arrival fuel, and the minimum departure and arrival fuels, helping to calculate safe points for the aircraft to continue or turn around if necessary. Factors such as aircraft weight and extreme weather make a difference to safe decision points.”

These calculations were then used for the Australia to Papua New Guinea flight, as the Air Force said storms often gather in the afternoons around Port Moresby.

Vincent said she had felt "nervous excitement" at the beginning of the flight, but added there were detailed preparations.

When we got to Papua New Guinea, there were a lot of happy faces. People were very excited to be the first Kiwi NH90 aircrew to fly from one country to another.”

“There was a celebration in the helicopter when the crew saw PNG for the first time,’’ she said.

“The success of the mission highlights the utility of the aircraft with long-range tanks which provided further deployment options for moving around the region when Navy ships or partner aircraft like the C-17 Globemaster are unavailable,” Vincent said.

The Air Force confirmed crews had since returned to Australia, and will continue to be based out of the Royal Australian Air Force Townsville. Plans for the aircraft to return to New Zealand were being finalised.