An independent investigation into the shooting of an armed gunman by a police officer has found that the officer's actions were justified.

The Independent Police Conduct Authority has released the ruling details into the December 2017 incident.

Authorities were called to a campground in the Manawatu after reports of a man behaving erratically and who was threatening his partner with a sawn-off rifle.

Other people at the campsite called police and the Armed Offenders Squad was called out.

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The details show that four AOS officers only approached the man when he entered a campervan where several campers had taken refuge.

"He threatened the owner of the campervan and made him drive towards police.''

The campers inside managed to escape, just as the man involved took the driver's seat and aimed his sawn-off rifle at one of the officers and then at a dog handler at the scene.

"The first officer fired at the man, hitting him in the arm.''

Authority chairman Judge Colin Doherty said: "Police were justified in firing at this man and it was reasonable for them to do so in the circumstances.

"Similarly, deployment of the police dog was justified. The man has since pleaded guilty to multiple charges relating to this day, including charges of using a firearm against a law enforcement officer."

The authority also ruled that the officers had armed themselves in accordance with police policy. They had also exercised appropriate command and control over the incident.