The public should continue to keep children and dogs away from beaches in Auckland's Hauraki Gulf while the source of a mystery toxic poison is investigated.

Five dogs have died and 14 fallen ill after visiting the beaches.

Auckland Regional Public Health Service (ARPHS) said tetrododoxin, a potentially fatal and extremely poisonous neurotoxin found in puffer fish, has been found in dead sea slugs and the vomit of one dead dog.

The latest dog died after exercising at Stanmore Bay during the weekend.

Government scientists were investigating the death.

"We don't know the cause yet, but this new death may be linked to death and illness in other dogs," ARPHS clinical director Dr Julia Peters said.

ARPHS's advice to the public:

- Children should not be taken to Hauraki Gulf beaches;

- dogs should not be taken to the beaches;

- other visitors to the beaches should not handle any marine life;

- people should not swim at beaches, but swimming off boats is considered safe; and

- people should not collect shellfish from the beaches.

"We know that going to the beach is a favourite pastime for Aucklanders so these warnings are not made lightly," Dr Peters said.

"We understand people's frustration at this constraint on their lifestyle but our overriding concern is to protect public health."

Poisonous sea slugs were a new risk and the service was concerned for people's safety, Dr Peters said.

"If a child eats or puts one of these creatures in their mouth it could be potentially fatal."

Sea slugs live on the sea bed. They are found in shallow and deep water and may be washed up.

It was not known how the slugs collected from Narrow Neck beach had acquired the poison.

Anyone exposed to sea water or life who develops symptoms within one to two hours should seek immediate medical attention.

There are no restrictions on recreational fishing at the beaches.