An Air Force Orion as been sent to search for a Tongan fisherman whose boat suffered engine failure during a fishing trip on Saturday.

The Defence Force says the Orion has left Whenuapai in Auckland following a request from the Rescue Coordination Centre.

It's due to reach the main search area, off the main island of Tongatapu, about 4.45pm today (5.45pm Tongan time).

The fisherman rang his family on his mobile phone about 6pm on Saturday after the engine of his 4.8-metre boat failed - and hasn't been been heard from since.

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A search is already underway in the 16,000 square kilometre search area between the islands of Tongatapu and Eua.