A 2-year-old girl became ill five minutes after drinking seawater at Narrow Neck Beach the same day a second dog died after eating toxins there.

The Auckland Regional Council last night repeated its warnings to keep children and dogs away from Hauraki Gulf beaches after the death of five dogs and illness in at least a dozen others.

Devonport resident Deana Busch said her daughter Natasha was digging with a spade around rock pools at Narrow Neck when she took a sip of the water.

Within five minutes she started vomiting.

"For the next 36 hours she couldn't keep her food down," Mrs Busch said.

"She lacked energy for the next week. She'd definitely lost a bit of bounce in her step, it took her a week to come right."

Natasha also had diarrhoea.

Mrs Busch contacted her GP, who told her to keep her daughter at home and keep her fluids up.

She later discovered that two dogs had died at the beach - the second on the same day - and suspected Natasha had been affected by the same toxin.

Mrs Busch said her daughter was not usually a sickly child - she hadn't vomited once in the last 12 months. She had been in perfect health until the pair went to the beach.

She contacted the North Shore City Council's Actionline to report the incident.

Council spokesman Rob Pitney said council officials were aware of the incident, but it happened in the early stages of the investigation when they were "unsure what they were dealing with".

The council had had no other reports of people becoming sick at beaches.

Stomach contents of a beagle that died after walking on Narrow Neck in early July were sent to the Ministry of Agriculture and Forestry for testing.

But it was not until a second dog died three weeks later on August 3 after walking on the same beach that a public warning was issued.

Tetrodotoxin, a toxin found in puffer fish, was found in the dead dogs' vomit and in a sea slug sample taken from the beach.

Auckland Regional Council spokesman Andrew Bristol said samples from the fifth dog to die on a Hauraki Gulf beach - at Stanmore Bay on the Whangaparaoa Peninsula - expected this week would establish whether it was a victim of the same toxin.

Experts said last week that the toxin that killed dogs is deadly enough to paralyse humans in seconds and kill them within an hour if they ingest it.

Mr Bristol said yesterday people should not take children or dogs to Hauraki Gulf beaches, swim or touch any birds or marine life or collect shellfish until more is known.

The ARC is co-ordinating efforts by eight agencies to identify what is killing the dogs, and is also testing dead dolphins and pilchards washed up on Auckland beaches.

It is not known if the marine deaths are linked to the dog deaths.