Scientists will test dolphins and pilchards this week to find out if they are victims of the same toxin, found in puffer fish, as a dog that died on Narrow Neck beach.

Tests have found that tetrodotoxin, a poison found in puffer fish, killed at least one of the four dogs that died after walking on Hauraki Gulf beaches.

The same toxin, which is highly toxic to dogs and people, was found in sea slugs found on the beach near where at least two of the dogs died, but researchers do not know how the poison got into the slugs.

A spokesman for the Auckland Regional Council, which is heading efforts to manage the poison risk, said other dogs that died or became ill would also be tested for the poison.

So far tests had not established a link between the dog deaths and deaths of penguins or other sea life, said spokesman Andrew Bristol.

The slugs have now been identified as pleurobranchaea maculata - or grey side-gilled sea slugs.

An article on Sea Slug Forum, an information site run by the Australian Museum's Bill Rudman, said grey side-gilled sea slugs were "opportunistic carnivores and voracious feeders that actively foraged for food". The slugs ate a wide variety of animals, but seemed to prefer sea anemones, said the article.

Last week Dr Rudman said sea slugs had ways of co-opting chemicals from algae or sea sponges to make themselves taste bad, but he could not see how they could ingest enough to kill a dog.

Meanwhile, Herald readers reported seeing puffer fish washed up on beaches on Auckland's North Shore and Coromandel Peninsula.

Terry Graves said he found two puffer fish washed up on North Shore beaches in 48 hours.

A spokesperson for Auckland Regional Public Health Service said advice not to take children or dogs to Hauraki Gulf beaches, and not to swim off beaches, touch any birds or marine life or collect shellfish still stands.