NSW Police are investigating why a coal seam gas protester was sprayed in the face with capsicum spray while he was chained to a truck.

Police have issued a statement confirming the incident, which happened on Tuesday near Kyogle in the state's north.

A group of protesters have been attempting to disrupt test drilling by the company Metgasco at their coal-seam gas site near Kyogle since January.

On Tuesday one of the protesters, a 45-year-old man named Gareth locked himself to one of the company's trucks as it attempted to leave the site.

He was arrested but not charged.

Scott Sledge is the President of the protest group the Northern Rivers Guardians.

He says he was on the other side of the truck when Gareth was sprayed.

"He told me a policeman came under the truck and said 'let go now and get out from under the truck or I'll spray you in the face'," Mr Sledge said.

"[Gareth] said I can't let go I'm locked on..I heard him yell out..I've been maced! Then I yelled out, you can't do that, he's locked on, he's defenceless, that's torture."

Southern Cross University law lecturer Aiden Rickets liaises between protesters and police.

He says it appears the officer has gone too far.

"Clearly where a person's got both of their arms locked and they cant remove themselves, to spray them in the face with capsicum, you know, isn't just assault, it pretty much amounts to torture," he said.

"We would view that very very gravely and we would certainly expect that the senior police would investigate that and take appropriate action."

Police guidelines state capsicum spray can only be used under three circumstances: To protect human life; As a less lethal option for controlling people where violent resistance or confrontation occurs; Or as protection against animals.

The Lock the Gate alliance, which organised the protest, says it will be lodging a complaint against the police.

New South Wales Police have confirmed an internal investigation into the incident is underway.