A spike in the number of dead dolphins in the Hauraki Gulf could point to a problem other than poisonous sea slugs - if only authorities would do the necessary tests, says a researcher.

Marine biologist Dr Karen Stockin, who is responsible for autopsies on common dolphins that wash up around Auckland, wants authorities to investigate the deaths of eight otherwise seemingly healthy dolphins in the space of three weeks.

She said the deaths were probably not linked to dog deaths that occurred at about the same time, "but that does not mean we should not be concerned".

"The common dolphin is an important biological indicator and if something changes in their system that causes an increase in deaths, that is important."

Dr Stockin said that on average, one or two common dolphins would wash up around the coastline of New Zealand each month.

However, "eight independent deaths from one region [were reported] in just a three-week period", she said.

"Not a single [dolphin] was emaciated or malnourished. On the contrary, the animals examined were seemingly healthy prior to death."

Tissues tests had failed to explain why they died. "These were all mature, robust animals."

Dr Stockin, a Massey University scientist, is pushing for the dolphins' stomach contents to be tested.

She has kept samples from their stomachs and contacted the Auckland Regional Council - which has been leading the investigation into deaths of dogs and other marine life - to ask about testing them.

She was told it was "not a line of inquiry" the council wanted to pursue.

The council's monitoring and research manager, Grant Barnes, said yesterday that the priority had been explaining the dog deaths. "From our perspective we had to get on top of why the dogs were dying."

A taskforce of agencies started out testing many different species but narrowed that once dog deaths were linked to sea slugs found to be poisonous with tetrodotoxin, said Mr Barnes.

The council and other agencies ruled out a link between the dog and dolphin deaths without testing the dolphins' stomachs.

Dr Stockin said dolphins were often among the first victims of changes to their environment because they were high in the food chain. Whales and seabirds were other "indicator" species, she said.

Dolphins are the only species whose deaths have not been explained after a spate of initially mysterious deaths.

Pilchard beachings were attributed to a virus attacking their gills, several penguin deaths were blamed on malnutrition, while sudden dog deaths and illnesses have been linked to tetrodotoxin.

Mr Barnes said Dr Stockin was the best person to comment on dolphin deaths but "we don't believe there's anything to suggest acute changes in the Hauraki Gulf ... it is part of the natural life cycle".

He said it was up to the Department of Conservation to decide if the dolphins warranted further testing. A spokeswoman for the department did not return messages yesterday.

Dr Stockin was keen to test pilchards found in stomachs of some dolphins, after other pilchards were found to have died of a herpes virus.

"We have a number of seemingly healthy dolphins found with stomachs full of a type of prey that has been washing up dead ... We should be looking at that."