Blue-green algae, which is toxic to dogs and can make people unwell if they drink or swim in it, is about to bloom in Anderson Park pond, Napier City Council says.

It looks like spilled paint or oil at the water surface.

The council warned the public that contact with Anderson Park ponds and all urban waterways should be avoided by humans and dogs.

"As we cannot confirm whether this has affected other waterways, all urban Napier waterways should be avoided by dogs and humans," council said in a Facebook post.

If a dog comes in contact with the water and is unwell it should be taken to a vet immediately.

While this is an issue specific to Napier's Anderson Park, it is a timely reminder that this is the peak time of year for these blooms.

Symptoms in dogs include: vomiting, diarrhoea, seizures or fits, weakness or collapsing, unconsciousness, disorientation or confusion, drooling and breathing difficulties.

There have been two reports of dogs falling ill after swimming at Anderson Park, the council said.

Blue-green algae can occur in stagnant water in river, streams, ponds or lakes, and slow-flowing and nutrient-rich waterways which experience consistently high temperatures for an extended period of time.

How long it will last depends on the weather.