Environmentalists are threatening to call a boycott of New Zealand's billion-dollar seafood export industry unless the government boosts efforts to save the world's rarest dolphin, which has dwindled to a population of 50.

The Maui's dolphin, the world's smallest and scarcest sub-species, is found only in shallow waters off the North Island of New Zealand.

It is listed as critically endangered and scientists from the International Whaling Commission (IWC) have called for urgent action to prevent extinction, including a ban on fishing in the dolphin's habitat.

While the government has taken some steps, environmentalists say it has not gone far enough due to fears of damaging the fishing industry, making a boycott the only way they can get their message across.

"We've exhausted all other avenues of making progress," Barbara Maas, an endangered species specialist with conservation group NABU International, told AFP. "We've repeatedly argued the scientific merits of the case and been ignored.

"So if it's all about money, as has become apparent, then we need to change the economic landscape in order to make them reconsider."

Maas said a seafood boycott would lead to customers questioning the "100 percent pure" branding that New Zealand prides itself on.

"The shame of this is that it will besmirch the environmental reputation of the country and once you start that process it's difficult to turn it off," she said.

"It would have been simpler and easier and cheaper to sort this out when the international scientific community raised the issue with New Zealand and said it needed to take action urgently."

- 'Situation desperate' -

New Zealand's seafood exports are worth more than NZ$1.5 billion ($1.2 billion) a year, according to official data, with China, Australia the European Union and United States the largest markets.

Maas said more than 100 conservation groups worldwide have signed up to support the boycott, which she said was set to be imposed within months.

"The last thing we want to do is cause grief to the people of New Zealand or even the New Zealand fishing industry, but the situation is desperate," the Adelaide-based head of Whale and Dolphin Conservation Australasia Mike Bossley told AFP.

He said a boycott would encourage customers not to buy New Zealand seafood and call on retailers not to stock the product.

Seafood New Zealand chief executive Tim Pankhurst said the government had already banned trawl and set-net fishing in 6,000 square kilometres (2,300 square miles) of waters and there was no evidence Maui's dolphins existed outside this area.

"Suggestions that consumers avoid New Zealand seafood to protect Maui's dolphins are accordingly misplaced," he said.

A spokeswoman for Conservation Minister Nick Smith said there had been no sightings of Maui's dolphins in the extended area where the IWC wants a ban imposed.

"If there is a credible sighting of a Maui's dolphin beyond where we have protections in place, then we'd be happy to initiate a review," she said.