The overtime bill for policing the oil protest at Woodburn Forest has soared to almost £140,000, the Belfast Telegraph can reveal.
Campaigners opposed to petroleum drilling close to a drinking water reservoir believe the total policing cost may reach £1 million if fuel, vehicles and administration costs are taken into account.
The PSNI said officers have racked up an overtime cost of £138,457, following a Freedom of Information request.
Campaigners have branded the PSNI "private security for Infrastrata", the company which is carrying out exploratory drilling for petroleum.
Environmental pressure group Friends of the Earth (FoE) said it was disturbing that considerable resources are being deployed by the PSNI to provide what is effectively private security for Infrastrata.
Northern Ireland FoE director, Dr James Orr, said: "On virtually every level there has been a breakdown in the rule of law. We have made repeated attempts to explain to the police the legal uncertainties around this project.
"Instead of upholding the rule of law, the PSNI seem intent on preventing the local community from exercising their lawful rights to use rights of way in Woodburn.
"We should not forget that this is the only drill in the world in a protected water catchment and there is no planning permission."
Dr Orr said the figures relate only to overtime costs before the drill came in and are a fraction of the total policing costs.
He added: "A conservative estimate is that the costs are up to £1m. This is a huge strain on the public purse, never mind the distress caused to the community, the annexation of a public forest and the risks to our drinking water."
PSNI Chief Inspector Stephen McCauley said: "Our priority continues to be ensuring the safety of all concerned at the Woodburn site whilst trying to keep everyday life running as normally as possible around it. "We are constantly reviewing the resourcing profile of police officers and the police numbers in this area.
"However, we are committed to using whatever resources are necessary to police this protest.
"We will continue to ensure that we have enough resources to respond to all eventualities. "Police remain at the location to ensure there is no breach of the peace and to facilitate peaceful protest. While the majority of protest activity to date has been well within the law, the recent actions of some protesters have gone beyond what can be accurately described as 'peaceful and lawful protest'.
"Where offences are being committed or where actions may lead to a breach of the peace, it is incumbent upon police to intervene and address these issues as they occur.
"As a police service we respect the rights of people to protest as long as it is within the parameters of the law."
Infrastrata estimates drilling will take around six weeks.
Earlier this month, NI Water revealed that two reservoirs supplying drinking water had been put out of service while oil exploration is carried out nearby. The protesters have been backed by Avengers star Mark Ruffalo and human rights advocate Bianca Jagger.
Last week, maths tutor Mark Chapman chained himself to the drill rig with a bicycle lock as it moved towards the site, but later left it of his own accord.
He was arrested on suspicion of obstruction and released on police bail.
Mid and East Antrim Borough Council faces a legal challenge taken by a local resident.