At least two more dogs have died after becoming ill shortly after being walked on Thames Coast beaches.

On January 12, a chihuahua died soon after being walked on the beach at Ngarimu Bay and two other dogs were earlier treated with similar symptoms.

The chihuahua's vet suspected the dog had come in contact with a toxic sea slug that could cause dogs to become ill within 30 minutes of being touched or eaten.

Delwyn Dickey The toxic sea slug is suspected to have killed two dogs and made other dogs ill on the Thames Coast.

Te Mata Bay residents Jannine​ and Ray Kelly said their dog Nuggitt also died not long after going for a swim at the beach.

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Ray Kelly said their healthy, trim 11-year-old labrador became restless and refused to eat shortly after swimming in the Firth of Thames, though he didn't show any signs of pain.

Supplied Karla Reeves' treasured Chihuahua also died just thirty minutes after walking on a Thames Coast beach.

"After swimming, Nuggitt went off his food straight away," he said.

"I'd been fishing and usually give him bits of fish bits during filleting and he didn't even eat them - and he's a labrador and he always eats them."

Jannine Kelly said Nuggitt spent the rest of the day seeking out cool places to lie down.

"We just thought he was hot from the heat of the day but obviously it was something else," she said.

"That night, he normally has his night biscuit but we realised in the morning that he didn't even eat that biscuit."

Nuggitt spent the night walking around the property.

"He'd been restless all night, walking up and down the driveway.

"He had gone missing in the night and I went looking for him before I went to work and couldn't find him. "

Nuggitt was found dead later that day.

"He just tucked himself under the flax bush and died. I'm still gutted, I'm not over it yet," she said.

A neighbour had also lost their dog after becoming ill following a swim at Tapu beach, with similar symptoms, she said.

Both dogs had blood coming out of their mouths after they died, she said.

The couple did not get an autopsy so were not sure what caused Nuggitt's death, but wanted to warn other dog owners.

"It just seems very coincidental that dogs are suddenly dying at the same time," she said.

"It's just about making people aware that there is something there."

A Ministry for Primary Industries spokesman said sea slugs were a native species that had a naturally occurring toxin which could affect dogs. It was up to local councils to respond to the issue, he said.

A Thames Coromandel District Council spokesman said no dog deaths or illnesses had been reported to council.

"If any vets came to us with evidence of dogs being poisoned by eating sea slugs we would investigate," he said.

"If people are being made sick, the District Health Board should be contacted and then we would work with them and probably also Waikato Regional Council."

In 2009, the potent tropical poison tetrodotoxin was found in sea slugs on Hauraki Gulf beaches and it's thought at least a dozen dogs had died after eating the dead slugs. Tetrodotoxin is also extremely toxic to humans and even a very small dose can be fatal.