Toxic algae are being treated as a likely cause of illness in dogs walking on the beach - though health authorities initially thought they were dealing with a malicious poisoner.

A spokeswoman for the Auckland Regional Public Health Service said officials believed a dog-poisoner was at work when they first heard reports of sick dogs, two months ago on Kohimarama beach. The dogs had been seen eating something above the high-tide mark so natural toxins seemed unlikely, she said.

Reports of sick dogs in Blockhouse Bay parks last year turned out to be deliberate poisoning with laced sausages.

A dog-poisoner was again suspected when two dogs were reported dead at Narrow Neck beach, said medical officer of health Denise Barnfather.

It was not until health officials heard of sick dogs at Waiheke and Browns Bay and deaths of other wildlife, on Monday, that they suspected something was in the seawater.

About the same time, the Department of Conservation connected the dog deaths to increasing numbers of dead penguins in the past week.

Scientists are trying to find the link, if any, between the dog deaths and unusually high numbers of dead dolphins, penguins and pilchards in the Hauraki Gulf.

Tests on the stomach contents of two dogs that died did not show algae but researchers are awaiting further tests on one of the dogs, penguins and the stomachs of dolphins.

Jellyfish and seaweed from beaches near where the two dogs died are also being tested. The first test results are expected back on Monday.

Herald reader Andy James reported to the Department of Conservation of seeing up to 40 dead little blue penguins in the past two weeks on beaches north of Auckland at Mangawhai, Pakiri, and Te Arai.

Russell resident Alan Wilkinson found five dead little blue penguins at Brick Bay and nearby Tapeka Bay on Tuesday and Wednesday.

Another reader reported seeing eight large jellyfish, possibly Lion's Manes - a large pink-red creature whose venom can affect the respiratory system - at Takapuna yesterday.

However, Michael Beasley, from the National Poisons Centre, said jellyfish stings would be unlikely to cause the kind of symptoms reported in the ill dogs. The medical toxicologist suggested algal poisoning was more likely, although he stressed he was not an expert in the area.

He listed several symptoms of algal poisoning matching those of the dead and ill dogs. People and dogs can become ill if they eat shellfish or other marine animals that feed on toxic algae.

One reaction to such toxins - which shellfish can accumulate without being harmed - includes weakness and paralysis of limbs, nausea, vomiting, diarrhoea and breathing difficulties.

Another is gastrointestinal symptoms and, occasionally, seizures.

The Ministry of Agriculture and Forestry is believed to be treating toxic algae as the most likely cause of the dog deaths, although a spokeswoman stressed that the only cause ruled out was 1080 or Brodifacoum poisoning.

The third of three drops of the rat poison Brodifacoum went ahead on Rangitoto and Motutapu islands yesterday.

The stomach contents of five dead dolphins found in the Hauraki Gulf - many full of pilchards - have been frozen for testing once MAF has carried out virus testing on pilchards that have washed up.

The dolphins' stomachs will be tested for toxic algae at the same time.

* If you come across anything unusual on the beach please email newsdesk@nzherald.co.nz