New Plymouth ratepayers may have to pay for the costs of an oil discharge into the wastewater treatment plant.
The discharge is believed to have entered the sewerage reticulation system in the industrial area of Bell Block on Friday.
New Plymouth District Council Water and Wastes manager Mark Hall said the substance, which could be drilling waste, was deliberately dumped into a manhole.
Electricity consumption spiked over the weekend to treat the oil.
"To put into context, we've got four blowers on site. Normally we use one, or one-and-a-half. We had all four blowers going at 100 percent. And that's going to come in at quite a significant cost to the rate payer unfortunately," Mr Hall said.
The substance, which could be drilling waste, was deliberately dumped, he said.
A suction truck was able to retrieve about 5000 litres of the material at the manhole but many more thousands of litres have made their way into the system, which could be affected for at least a month.
Mr Hall said only the quick actions of his staff and the use of a regional council boom prevented oil getting into the sea.
Mr Hall said the final cost could be up to $10,000 or more - and that would probably be paid for by rate payers if the culprit was not found.
