A council spokesman said observations showed toxic algae cyanobacteria blooms present at Upper Tomahawk Lagoon in Dunedin.
A warning sign was installed at the site yesterday to advise avoiding contact with the water at the lagoon.
The algae appeared as murky green water, like pea soup.
He said to look for dark brown mats like tar on the riverbed — they could detach and wash up in the shallows on the banks.
Its musty smell was attractive to dogs and, if eaten, could result in severe poisoning or death.
If people or pets had been near the water at the lagoon, symptoms to watch out for included.—
• Skin rashes, nausea, tummy upset, tingling and numbness around the mouth or tips of fingers.
• Signs of poisoning in animals include lethargy, muscle tremors, fast breathing, twitching, paralysis and convulsion.
The spokesman said if someone or their pet experienced the symptoms, they should visit a doctor and/or vet immediately.
