The New Zealand Defence Force (NZDF) is sending one of its aircraft to Tonga to help in the search for a fisherman who hasn't been seen in two days.

Search crews have been experiencing rough seas in their search for the Tongan fisherman off the island of Mo'unga'one in the central part of the country.

Group Captain Nick Olney, the Acting Air Component Commander, said a Royal New Zealand Air Force P-3K2 Orion aircraft flew out this morning after a request from the Rescue Coordination Centre New Zealand (RCCNZ).

Olney said the fisherman left Mo'unga'one Island about 5am on Tuesday in a 6m fibreglass boat to pick up five other fishermen from nearby Nukupule Island, about 16km away.

"He has a mobile phone but no communications and safety equipment on board," Olney said.

"We will do our best to search for him and we are hoping for a positive outcome."

RCCNZ said a local policeman had advised the fisherman to postpone the trip because of the rough conditions. About two hours later the fisherman rang the cellphone tower in Tonga's Pangai Island asking for help because his boat had broken down.

The search, which began on Tuesday, has involved local fishing vessels and a patrol boat from Tonga's armed forces. A commercial aircraft that flies regularly between Tongatapu and Vava'u islands was asked on Wednesday to keep a lookout for the boat but has not seen it.

Local radio stations have also asked fishing vessels in the area to be on the lookout for the missing fisherman.

The Orion is expected to reach the search area about midday.