Swimmers are warned to avoid the Hutt River below Kaitoke, as there's a risk they could swallow potentially fatal toxic algae.
Officials are advising people to keep clear of the river and say the algae is "blooming" in the warm, dry weather.
"Hot weather, warm water and a lack of rain in the last two weeks have made conditions ideal for the native algae to suddenly bloom," said environmental scientist Dr Mark Heath.
More Algae is breaking off rocks and flowing down the river than normal, endangering both swimmers and dogs.
"Even very small amounts of toxic algae, if swallowed, can cause serious illness and - although unlikely - can potentially be fatal."
The Greater Wellington Regional Council is advising dog owners to keep them well away from the water and on a leash.
Dr Heath said the algae gives off an earthy and musty smell that attracts dogs.
At this stage, the warning is only in place for the river below Kaitoke, but other rivers in the region are being closely monitored.
It's hoped that a burst of heavy rain will flush out the river and reduce the risk of harm.
The symptoms of algae poisoning are nausea, vomiting, numbness, tingling, muscle twitches, shaking, weakness, breathing difficulties, and - although unlikely - potentially convulsions and loss of consciousness, Dr Heath said.
Anyone who experiences these symptoms should see a doctor ring the health line on 0800 611 116.