Passengers and crew on a dolphin-watching cruise near Tauranga have been treated to a rare sighting of a whale shark.

The whale shark is the world's largest fish and can grow up to 18 metres long.

Dolphin cruise company Bay Explorer owner Brandon Stone told Morning Report the whale shark that cruised close to the side of his vessel on Tuesday was about 10m, but still presented a spectacular sight.

He said, although the northeast current through the Tongan trench brought the majestic creature into coastal regions off places like Hawke's Bay, Bay of Plenty and Northland each year, it remained rare to spot them on the surface. The encounter lasted about 30 minutes.

Although whale sharks are peace, plankton-eating giants, no one on the boat acted on the urge to swim with it.

"We didn't attempt to swim with it, we left it to do what it wanted to do and it was quite inquisitive. It kept swimming towards the boat and coming alongside, it was a great spectacle for the crew."

Source: RNZ.co.nz