Auckland Council is understood to have identified the culprits who dumped 28 barrels - some filled with used oil - at two car parks on Piha Rd in January.
It is believed a company, a company director and an employee have been the focus of a council investigation and charges are being laid under the Resource Management Act.
A council spokeswoman said the council was unable to give details at this stage.
Piha residents found an unpleasant surprise on January 3 after the barrels were dumped at the car parks. Some had tipped over and oil spilt on the ground.
At the time, a police spokeswoman said police had contained the spill but it could have been dangerous to locals.
"The spill has now been contained but could have run into the upper Nihotupu Reservoir which had the potential to contaminate the water supply," she said.
Piha resident Sarah Munro told the Herald locals thought somebody had removed distinctive features from the drums that could have helped identify their origin.
Piha, Karekare and Anawhata residents regularly called the council to have rubbish removed, but she said the oil incident was an all-time low.
"It's a major problem dumping rubbish at Piha - there is often household rubbish, furniture and even cars," Munro said in January.
