Basking shark sightings around New Zealand are much lower than they were even 20 years ago.

A new study has confirmed the small size of New Zealand's great white shark population, while also highlighting the rapid decline of basking shark numbers in recent decades.

The Department of Conservation report looks at the conservation status of 113 species of sharks, rays and chimaeras - also known as ghost sharks.

The status of great whites was classified as nationally endangered, with the total number of adult great whites in the waters of New Zealand and eastern Australia estimated at between 590 and 750. The total number including juveniles is estimated to be 5460.

PETER SCOTT A great white shark chasing tuna bait in Foveaux Strait. Few of the apex predators live in New Zealand waters. (file photo)

"White sharks, being apex predators, we always expected them to have a small population size," report lead author Clinton Duffy said.

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"It's undoubtedly lower than it was in the past for a few reasons. There would have been a lot more marine mammals - seals and whales - common around the New Zealand coast in pre-human times."

SUPPLIED Cage diving with great white sharks off Stewart Island.

Great whites were also taken fairly regularly as a by-catch by commercial and recreational fishers, and were caught in shark control programmes along the east coast of Australia.

While the seal population was gradually increasing, it was thought the level of by-catch, was probably enough to prevent the great white population from growing, Duffy said.

The first estimate of the great white population in New Zealand and Australia was only recently completed. It was a genetic estimate, using tissue samples, made by scientists from Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO).

SUPPLIED A basking shark, spotted near the Miramar Peninsula, Wellington, in 2003.

The CSIRO research bore out previous expectations that the great white population was either stable at a "very low" level, or in gradual decline, Duffy said.

"Basking sharks - we're a bit more at a loss about that species. We know very little about basking sharks and they do seem to have disappeared from coastal waters around New Zealand."

Basking sharks, which feed on plankton, could grow up to 12 metres long, making them the second biggest fish behind whale sharks.

Nowadays there were only sporadic sightings of basking sharks around the coast.

"It's relatively sudden. We know in the '60s there were regular stories of more than 100 of them off Kaikōura," Duffy said. As recently as the 1990s large schools were still being seen off the eastern South Island.

By-catch of basking sharks in the deepwater fishery had also dropped off to very low levels.

"Basking sharks have been known to do this - disappear en masse very suddenly in other parts of their range ... and suddenly reappear years later en masse again," Duffy said. "They are a very enigmatic animal, very difficult to study."

It was unknown whether there had been some sort of global, or Pacific-wide population shift, but the worldwide population estimate of fewer than 100,000 mature basking sharks was "very low."

"New Zealand is their Southern Hemisphere hot spot and the disappearance is concerning."

In the past they had been heavily fished in some parts of the world, and it was thought populations had been quite seriously reduced.

"There are signs of a recovery in the North Atlantic... Possibly all of the New Zealand (basking) sharks are now in the North Atlantic."

Overall, the signs were that New Zealand was doing all right in its management of the species of sharks, rays and chimaeras for which information was available.

"The big unknown is that third of fish considered data deficient. We don't know enough about them to say whether the trend is up or down," Duffy said.

There was evidence some species were being affected by coastal development, particularly a loss of nursery areas. That could include commercially-fished species rig and school shark.

While they were doing all right, school sharks had disappeared from some areas of the Hauraki Gulf where they used to be abundant.

As a result of new information, the status of four sharks covered in the report has been improved, compared to previous studies, to not threatened.

Those are the Galapagos shark and Kermadec smooth-hound, which are protected within the Kermadec Islands Marine Reserve, and six-gill shark and Pacific sleeper shark, which are widespread deep-water fish.

"Just under half of New Zealand sharks, rays and chimaeras, 55 species, are categorised as 'not threatened' with large stable populations," Conservation Minister Eugenie Sage said.

"But like much of the biodiversity in our vast marine area, our knowledge is patchy for this group; for over a third, 42 species, we don't know enough to assess their status."