Barrels of oil were dumped in two West Auckland car parks, near bush walking tracks.

Twenty-eight barrels of oil were dumped in the Waitākere Ranges overnight in an incident that could have contaminated the region's water supply.

Sarah Munro was passing the area about 8am on Wednesday when she saw dozens of barrels of oil and oil filters scattered near the entrance to a walking track off Piha Rd in West Auckland.

Puddles of oil surrounded the barrels, and oil had leaked out and seeped into puddles from the overnight rain.

NZ POLICE Police were investigating the incidents.

Police said barrels had been dumped at two carparks and they were investigating the incidents.

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"The spill has now been contained but could have run into the upper Nihotupu Reservoir which had the potential to contaminate the water supply."

NZ POLICE Police said the spill could have contaminated the water supply.

Munro took a photo of the 44-gallon drums to upload online and found residents fuming on local message boards about the incident.

Munro said people often dumped household waste and furniture at the carpark but this was the first time she had seen oil barrels and oil filters.

"This is an all-time low."

NZ POLICE Piha volunteer fire brigade members put absorbent on the spill to prevent it running into the bush.

From the way the barrels looked at the scene, Munro said she thought someone had pushed the barrels off the back of a truck.

Some of the barrels had hit the ground upright while others were lying on their side leaking oil, she said.

The carparks are nestled within the Waitākere Ranges where a rāhui, or restriction, has been put in place by iwi to prevent the spread of kauri dieback disease.

Police said they were searching for forensic evidence at the scene but also asked for anyone who saw any suspicious behaviour to contact them as soon as possible.

"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."

Piha volunteer fire brigade said in a Facebook post members also called out to the oil spill.

"All we could do is put some absorbent down to stop the oil running into the bush."

Police said anyone with information could contact Constable Kelly Johnson on 021 192 3033 or call Crimestoppers anonymously on 0800 555 111.