Greater Wellington Regional Council has been notified of a potential paint spill in Wellington Harbour.
Posts to social media show a white substance near the shore at Balaena Bay.
Greater Wellington spokesman Stephen Heath said they are still trying to understand the nature of the spill.
"In cases like these it's often the case that decorators are pouring waste into the stormwater system. In which case we try to identify the perpetrator. But we've yet to establish what's going on."
A pollution response team will be dispatched to the area in the next hour.
Roseneath resident Catarina Gutiérrez said she could see the substance from her house.
"It looked like the water on the waterfront was changing colour and I didn't know if it was the light or something in the water like an algae or something but then it started to grow."
Gutiérrez said there is a wastewater exit in the area which could be causing the spill.
She told the Herald that she was confident the substance was paint.
"It was a pretty strong white colour that was headed towards the point [Point Jerningham]."