A mystery fish videoed by a Kapiti diver has New Zealand and overseas fish experts baffled.

The diver, Ben Knight, who is also chairman of the Guardians of Kapiti Marine Reserve said, "I was out for a snorkel in the marine reserve and spotted what I thought was a small juvenile red moki with unusual colour bands.

"I was curious to know more about this fish so I took a short video clip of it and shared the video to our Guardians Facebook group.

"We have a number of marine fish experts within the group and they were all very excited to see the video.

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"It turns out this is not a red moki or any other common New Zealand fish and may in fact be a previously unidentified species which is pretty exciting."

He said a range of national and international marine scientists had looked at the footage and had tentatively identified the fish as a member of the cheilodactylidae family, which is a group of fishes commonly known as morwongs, that includes common local fish such as red moki (cheilodactylus spectabilis) and tarakihi (nemadactylus macropterus).

About 20 species of morwong are known from the southern Atlantic, Indian and Pacific Oceans, however the colour pattern of the Kapiti specimen is distinctive and does not match any of the known species, he said.

"Some fish experts have suggested that the fish may be a hybrid between tarakihi and red moki, however others believe this is unlikely.

"No hybrids between these fish, which are classified in separate genera, have ever been recorded."

Clinton Duffy from the Department of Conservation supported the tarakihi red moki hybrid theory based on the colour pattern and body shape of the fish.

"Hybridisation is a relatively well-known phenomenon in terrestrial species and is increasingly being recognized in marine fishes, even in sharks.

"This find is a great example of how life in the underwater world still holds many surprises, even for scientists."

Retired fish taxonomist and Guardians of Kapiti Marine Reserve trustee Chris Paulin thought the fish represented an unknown, undescribed exotic species that had probably arrived into New Zealand waters recently and survived as a result of the unusually high sea temperatures this summer.

He believed its origins may lie in an un-sampled seamount in the Indian Ocean.

"Sub-tropical and temperate water exotic species frequently turn up in New Zealand waters.

"Many have pelagic larval stages that drift for up to a year in ocean currents, or are carried in ship ballast water.

"Influxes of sub tropical species often are observed following cyclones which push large volumes of surface water, carrying larvae, south into New Zealand.

"There are more than 150 exotic marine species in New Zealand's coastal waters already, and at least one new species arrives every year according to a NIWA report.

"If conditions are suitable, many of these species are able to survive in New Zealand and grow to adult size, but not breed in the cooler water.

"It would be great if the local divers and fishers can keep an eye out for other, similar specimens outside of the reserve, that could be collected for more detailed study and genetic analysis to determine the identity of the mystery fish."

Whatever the identity of the fish, Mr Knight said the sighting highlighted the biodiversity values that Kapiti Marine Reserve protects and the important role marine reserves play in providing a 'baseline' of unexploited habitat and species against which changes resulting from environmental influences can be measured.