There are between 57 and 75 Maui dolphins left in the world, according to the preliminary results of a new survey. That is up very slightly from previous estimates.

Adult numbers of the critically endangered Maui dolphin have risen slightly, according to the preliminary results of a survey carried out over the past two summers.

The survey, by the Department of Conservation, the Ministry for Primary Industries, and researchers from Auckland and Oregon State universities, estimates the population at approximately 63 adults, with 95 per cent confidence there are between 57 and 75.

This represents an increase from a 2010-11 survey that estimated the number at 55, with 95 per cent confidence there were between 48 and 69.

Environment Minister Maggie Barry said the stabilisation of numbers over the past five years could only be good news, and Primary Industries Minister Nathan Guy said the new figures were an encouraging sign that restrictions on fishing were having an effect.

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However, the World Wide Fund for Nature (WWF) said the Government still needed to do "more than the minimum" to save the dolphin from the very real prospect of extinction.

"Clearly the numbers are still extremely low," spokesman Chris Howe said. "Maui dolphins need to be protected throughout their range from set netting, trawling and risky marine mining activities."

According to WWF, research shows more than 95 per cent of unnatural Maui deaths are caused by entanglement and drowning in set net or trawl fishing, and experts estimate that about five are killed each year this way.

Barry said: "While these results are encouraging, there is no dispute the Maui population remains at a very low level, and the Government remains committed to ensuring their long-term survival.

"While it is impossible to count every dolphin in the sea, and this figure can only ever be an estimate, the survey was carried out according to rigorous scientific standards."

Guy said: "More than 1700 square kilometres off the west coast of the North Island have been closed to trawl net fishing since 2003, and over 6200 square kilometres closed to set netting.

"Since July 2012, there has been mandatory observer coverage on all commercial set net vessels operating offshore out to seven nautical miles in the Taranaki region.

"And since March 2014, there has also been increasing observer coverage on the trawl fleet north of Taranaki."

But Howe said greater efforts were needed. "It requires a genuine sanctuary from Maunganui Bluff to the Whanganui River mouth, including harbours, out to 100 metres deep.

"At the moment, less than half of their habitat is protected and it has pushed it to the brink of extinction."

The full report from the survey will be published next month and used to inform the review of the Hector's and Maui Dolphin Threat Management Plan, scheduled for 2018.