Medical Officer of Health Dr Derek Bell told the Otago Daily Times yesterday, while people were less sensitive to the cyanobacteria algae than dogs, they should still avoid it.
The Otago Regional Council has posted signs warning people to stay away from the river, where mats of the algae were discovered just over a week ago.
"I think the advice that has gone out is perfectly reasonable,'' Dr Bell said.
"It's absolutely impossible to quantify the risk because it's an unknown territory.''
The algae had previously shown up in Waihola, Tomahawk and Mataura waterways, Dr Bell said.
"I think this is the first time we have had a Central Otago river involved.
"But nationally it's not uncommon. It's probably a growing problem.''
There had been no "human-related health problems'' from the algae previously in New Zealand, but there had been problems overseas, Dr Bell said.
"Humans are not as sensitive as dogs are to the toxin and, additionally, they do not generally consume as much as the dogs do.
"Some dogs eat the mat in the water or on the riverbank and they also lick themselves down after being in the water so they consume much, much higher doses.''
The poison can be absorbed through the skin.
Symptoms include skin rashes, nausea, stomach cramps and tingling and numbness around the mouth and fingertips.
