The Central Hawke's Bay District Council says it is not sure what has made parts of the Tukituki River smell like a septic tank.
A group called Friends of the Tukituki says the smell has been around for several months and blames the sewage plants in the towns of Waipawa and Waipukurau, which discharge directly into the Waipawa and Tukituki rivers.
The council's technical services manager, Steve Thrush, says the new plant in Waipawa has been offline for adjustments since before Christmas, but it is still meeting the conditions of its resource consent.
He says the sewage plants are not the cause of the smelly pollution.
Mr Thrush says the cause could be sewage discharges from houses along the river, or rotting weed, but the exact cause is not clear.