Whanganui District Council is appealing for help from the public to identify who dumped a pollutant, thought to be paint, into a pond at Rotokawau.
The pond is above Virginia Lake in a reserve area in the Rotokawau subdivision.
Stormwater technical officer Brad Haburn said the council was notified about the incident on October 5 and maintenance contractors Veolia used a vacuum truck to remove as much of the polluted water as possible.
It's likely the pollutant was dumped directly into the pond.
"Council infrastructure staff inspected the upstream stormwater network but no signs of a dumped pollutant could be seen," Haburn said.