Crew aboard a cruise ship saved six men on a leaky Tongan fishing boat who had not had water for three days.

The men sent a distress call at dawn after they were unable to start the boat's motor due to a flat battery.

The Rescue Coordination Centre organised for German cruise ship Albatros to divert its course at 5am to reach those on the 11.5 metre fishing vessel, 400km off the coast of Tonga.

Senior search and rescue officer Ramon Davis said the boat had been taking on water and was unlikely to remain afloat for more than 24 hours.


"They had been in a truly life-threatening situation," he said.

The New Zealand Air Force dropped emergency supplies to the distressed men, shortly after reaching the search area around 9am.

Air Commodore Darryn Webb, the Air Component Commander, said an Air Force P-3K2 Orion surveillance aircraft left Whenuapai at first light this morning.

He said emergency supplies including a radio, water and food were dropped to the fishing boat.

"It's always pleasing to successfully locate those in distress in any search-and-rescue mission," Air Commodore Webb said.

"Although the vessel appears to have communication difficulties, the crew reported that the two men they spotted on board were safe and well."

When the Albatros finally reached the boat at 2.30pm today, the men jumped into the water and swam to the ship, where crew helped transfer them aboard.

The ship is now bound for Auckland.

Davis said that while the fishing boat was not carrying adequate radio equipment, its rescue beacon enabled the crew to call for help.

"We do however recommend all vessels carry at least two methods of signalling distress appropriate to their area of operation.

"We would like to thank the Orion crew and all on board the Albatros for coming to the rescue of these fishermen so promptly."

The Air Force's NH90 medium utility helicopters and the P-3K2 Orion surveillance aircraft flew around 230 hours on 18 search and rescue missions in New Zealand and the Pacific last year.

This was a 56 per cent increase above the 147 flying hours recorded in 2015.