Dolphin watchers spot world's largest fish near Tauranga

A boat full of dolphin watchers off Tauranga got a big surprise when they spotted the world's largest fish.

Passengers and crew on board the Bay Explorer first saw a fin and were lost for words when the fish - a whale shark - swam into their view.

The whale shark can be as long as a bus but despite its size and name, it's described as a docile creature that feeds on tiny plankton.

"Something very very special happened today at work and I don't think I'll ever forget this moment," Bay Explorer skipper Nik Weyel said in a Facebook post on Tuesday, accompanied with a video of the whale shark.

"Excuse my extreme excitement but I'm sure you can all understand."

According to the Department of Conservation (DoC) website, the whale shark only occasionally visits northern New Zealand's waters.

"The whale shark is the largest fish in the world, with reliable reports of some as long as 18m," DoC says.

"Whale sharks are found worldwide in almost all tropical and some warm temperate waters.

"They are highly migratory and aggregate seasonally in well-defined areas in response to increased productivity and food availability."