People are being asked to steer clear of the Otaki River, after dead fish were found near the river mouth.
Greater Wellington Regional Council officials are investigating.
They've collected carcasses and water for analysis.
The council says at this stage there's no obvious cause of death, and no sources of pollution or contamination found.
It's asking people not to fish, gather mahinga kai, or undertake recreational activities in the water in the estuary while tests are carried our.