Marine scientists have urged New Zealand to step up efforts to save the world's rarest dolphin, saying only a few dozen Maui's dolphins remain and immediate action is needed to prevent their extinction.

The Maui's dolphin, dubbed "the hobbit of the sea" and found only in shallow waters off the North Island's west coast, is listed as critically endangered, and the International Whaling Commission's (IWC) scientific committee said it was extremely concerned that fishing will entirely wipe out the sub-species.

"It is of the highest priority to take immediate management actions that will eliminate by-catch of Maui's dolphins," it said in a report published Tuesday.

"This includes full closure of any fisheries within the range of Maui's dolphins that are known to pose a risk."

The grey and white Maui's, named after a Polynesian demi-god, is one of the world's smallest dolphins, with a maximum length of 1.7 metres (5.5 foot).

An estimated three-to-four dolphins are accidentally killed as by-catch every year, a loss the IWC report said was unsustainable in a population so small that is believed to have dwindled to about 50 adults.

The report, which summarises the finding of an IWC scientific committee meeting held in Slovenia last month, noted that New Zealand had made efforts to save the creature.

But it called for a total ban on gillnet and trawl fishing, adding: "The current management situation falls short of that required to reverse the Maui's dolphin's decline."

It is the third year in a row that the IWC committee, which is made up of more than 200 marine scientists, has called for urgent action from New Zealand to save the dolphin.

This time the body asked Wellington to provide it with an annual progress report so it can assess what is being done.

Conservation group NABU International said the request amounted to a rap on the knuckles for New Zealand after it had failed repeatedly to implement requests to ban fishing throughout the dolphin's range.

"I think the scientific community has really fallen out of love with New Zealand over this issue," NABU endangered species specialist Barbara Maas told AFP.

"Current protection measures are an arbitrary mix of inconsistent and biologically meaningless fisheries exclusion zones."

Associate professor of zoology at Otago University Liz Slooten said it was difficult for New Zealand to lecture Japan about its whaling programme when it was doing so little to save the Maui's dolphin, despite ample scientific advice on the need to act.

"It's hard to believe that in a country like New Zealand we are taking such a long time to save a dolphin that's only found in our waters," she told AFP.

"The international community is waiting for N.Z. to do the right thing -- it's become really urgent now."