A father and son missing at sea off Tonga since Saturday have been found.

Air Commodore Darryn Webb, the Air Component Commander, says a P-3K2 Orion surveillance aircraft from the Royal New Zealand Air Force's 5 Squadron spotted the two waving their t-shirts from Hunga Tonga island, about 58km northwest of the Tongan capital of Nuku'alofa, about an hour into the search.

The duo failed to return from a day-long fishing trip off Tonga's Eua Island on December 10.

Crew from the Orion reported that the survivors appeared to be well and their blue wooden boat intact.

Darryn Webb the aircrew dropped the survivors emergency supplies consisting of water, food and a radio, and contacted the Tongan Navy to arrange for their rescue later this afternoon.

'We are very pleased with the successful outcome of this mission and for the survivors to be reunited with their family. Christmas is about being with your loved ones so I am sure this news is a great relief to their family, friends and community.

'Although we are thousands of miles away from those who need help, we remain ready to assist our Pacific neighbours if required.”

The Orion left Whenuapai at 8.45am today to scour an initial search area of about 3000 square nautical miles off Tongatapu, the largest of Tonga's three main island groups.

The Rescue Coordination Centre New Zealand sought the NZDF's assistance as local search efforts were not successful, says Darryn.

The mission to Tonga is the 12th search and rescue operation undertaken by the NZDF in the Pacific since January.

The Air Force's NH90 medium utility helicopters and the P-3K2 Orion surveillance aircraft have flown over 210 hours on 17 search and rescue missions in New Zealand and the Pacific since January. This represents a 44 per cent increase over the 147 flying hours recorded for 2015.

Missions to the Pacific account for about 87 per cent of total hours flown on search and rescue operations this year.