Greater Wellington Regional Council has issued a warning for the Hutt River after toxic algae in the area reached dangerous levels.
Dog walkers and swimmers have been warned to avoid the river south of Silverstream through to Birchville after algae levels rose through January.
"Algae mats in these areas are thick and starting to peel, which presents danger to swimmers and dogs," says Greater Wellington environmental scientist Dr Mark Heath.
"We have to be prudent in these circumstances and advise the public that it's not safe to swim in those stretches of the river."
The algae can cause nausea, vomiting, numbness, tingling, muscle twitches, shaking, weakness, and breathing difficulties.
It is also possible for the algae to cause convulsions and loss of consciousness, although this is unlikely.
The council does not expect the rain forecast for Thursday and Friday to wash away the algae from the river, and is encouraging swimmers to check online before they go swimming around Wellington.
Those wanting to cool off in the heat will be grateful to hear the Akatarawa River and the rivers in the Kaitoke Regional Park are still safe for swimming.