Police are investigating the dumping of dozens of barrels of oil in the Waitakere Ranges near Auckland.
About 30 barrels containing dirty oil filters were scattered at two carparks on Piha Road.
Several barrels were dumped near the Upper Nihotupu Reservoir.
Police said they were looking for those responsible.
Police said the spill had now been contained, but could have run into the reservoir and contaminated the water supply.
Scene of Crime Officers have been called in to search for forensic evidence, but police have also asked for anyone who saw any suspicious behaviour to please contact them.
"This type of behaviour is extremely concerning and police are keen to hear from anyone who witnessed any suspicious activity, even if it was minor, we want to hear about it," said a post on the North Shore, Rodney and West Auckland police Facebook page.
Auckland Council was also looking into the incident.
Spokesperson Adrian Wilson, said staff were working to absorb the oil and clean up what they could.
He said pollution contractors were working to contain any spillage that may get into nearby waterways.