A police investigation has found officers in Northland trespassed on a man's property, then unlawfully pepper sprayed him and arrested him.

The Independent Police Conduct Authority oversaw a police investigation into the incident, the outcome of which was dealt with confidentially.

"The police investigation found that the officers who first visited the complainant were trespassing, as they failed to leave when the complainant made it clear that they were no longer welcome on his property," the IPCA summary said.

Pepper spray, or OC spray, was then used.

"Police found that the use of OC spray against the complainant was unlawful, because the officer was trespassing at the time; he should have left the property immediately instead of using force against the complainant."

The officers then left and came back with their supervising officer and unlawfully arrested the man for assaulting police.

"The arrest was found to be unlawful because officers were trespassing when the alleged assault occurred, and were therefore not acting in the lawful execution of their duty at the time.

"The actions and decision making of the officers involved were dealt with by way of a confidential employment process. The authority agrees with the outcome of the police investigation."

It's not clear if the officers were charged or sanctioned