A "huge" whale shark was the last thing two mates out on their jet skis fishing near Whangamata expected to see.

On Sunday afternoon, Alastair Shorten and a friend had driven about 15 minutes offshore from Whangamata towards the Aldermen Islands, where they stopped for a bit of fishing.

Soon after, Shorten heard a great splash behind him.

"I quickly turned around and saw a huge whale shark.

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"It took us a little while to realise what it was. I had never heard of them being in New Zealand waters, and thought it was something more ominous."

The distinct markings and giant, wide mouth gave its identity away.

"It was just really curious about the jet skis, and was swimming between them.

"It was pretty exciting - and made up for catching no fish. It is not what you expect to see on a Sunday afternoon."

The whale shark is the largest fish in the world.

The whale shark is the world's largest fish, with reports of specimens up to 18m long. File photo / Tourism Western Australia
The whale shark is the world's largest fish, with reports of specimens up to 18m long. File photo / Tourism Western Australia

The largest measured specimen is 13.7m long, but there are reports of whale sharks measuring up to 18m.

They occasionally visit northern New Zealand waters from November to March, with most sightings in February.

The Department of Conservation says those found in New Zealand have been between 3 and 15m.

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Shorten estimated this one to be about 6m.

"It was twice as long as my 3m jet ski."

He had never seen one before, and had even gone on an unsuccessful whale shark tour in Thailand several years ago.

"It is pretty exciting to be able to see one in our own backyard."

Despite their giant mouths and thousands of teeth, the filter feeders pose no threats to humans.

Their diet includes zooplankton, squid, fish eggs and small fish including sardines, anchovies, mackerel and young tuna.

Cracking The Mystery Of The Elusive Whale Shark. Source: AP

They are highly migratory, and are found in almost all tropical and some warm temperate waters.

There have been unconfirmed reports in New Zealand as far south as Timaru, and off Milford Sound, during a period of exceptionally long, warm summers in the mid to late 1970s.

Whale sharks are generally solitary but loose aggregations of more than 100 have been observed.

A group of jet skiers had another whale shark sighting at the beginning of March near the "Hole in the Rock" on Motukokako, off the coast of the Bay of Islands.