A poison found in tropical puffer fish, which is potentially fatal to humans, is to blame for the deaths of two dogs on Auckland beaches.

The toxin, tetrodotoxin, was also found in the dead dogs' vomit and also in a sea slug sample on Narrow Neck Beach on the North Shore.

The finding last night prompted public health officials to reiterate warnings that animals and children should not go to Auckland beaches.

"This substance is extremely toxic and potentially fatal to humans and animals," said a spokeswoman for Auckland Regional Public Health.

"It is not known how the slug came to contain the toxin and investigations are continuing. While evidence of the toxin has not been found on other beaches it is too early to rule this out."

Eight agencies have spent almost two weeks investigating the deaths of two dogs and illness in more than a dozen others.

The Cawthron Institute, which carried out the tests, said monitoring of Auckland's east coast and Coromandel beaches found evidence of dead sea slugs only at Narrow Neck and Cheltenham beaches.

"This tends to suggest that the issue could be localised," it said.

The ARC will continue to monitor Auckland beaches for sea slugs.

Auckland Regional Public Health Service said its advice was:

Dogs and children should not be taken to Hauraki Gulf beaches.

Other visitors to the beach should not handle any marine life, in particular any dead marine life or birds.

People should not swim from beaches but swimming off boats is considered safe.

People should not collect shellfish from beaches within the Hauraki Gulf.

It is not advising any ongoing restrictions on recreational fishing.