A Northland man has good cause to question whether his dog truly is his best friend after the canine leapt on to a loaded rifle and accidentally shot him in the backside in a freak, but potentially fatal, accident.

Police believe the dog may have accidentally stepped on the trigger of the .22 rifle involved in the shooting of a 40-year-old Te Kopuru man on Saturday afternoon.

The man, a back seat passenger in a 4WD, was shot in the buttocks when the dog jumped on to the loaded gun on the vehicle's back seat, at a Te Kopuru property, about 13km south-east of Dargaville.

The injured man was among a group of four who had just finished killing and butchering a domestic pig.

The group had completed the job and were climbing into the vehicle to leave when the gun went off.

The man was flown by the Northland Electricity rescue helicopter to Whangarei Hospital where he is reported to be in a stable condition. The bolt action rifle was loaded with a magazine containing five shells - only one had been used the kill the pig and the party told police they thought the gun was unloaded.

Dargaville police Senior Constable Ian Anderson said the man was very lucky not to have been more seriously injured and he warned of the need for vigilance with firearm safety.

"It should be standard procedure to ensure the bolt and ammunition are removed before transporting a firearm," Mr Anderson said.

He said the rifle would be tested to see it there were any faults which might have otherwise caused the firearm to discharge. It was believed a dog may have stood on the gun, causing it to fire.

Police have not yet spoken to the victim and will following up investigations with others present at the scene.

Acting St John Northland operations manager Mark Going said Dargaville paramedics were called to treat the man about 3.50pm on Saturday. They were told the gun had fired through the seat and lodged a bullet in the man's left buttock.

Mr Going said the man was not bleeding too badly but was in extreme pain.

A doctor at the scene of the accident helped the man before he was flown to Whangarei Hospital by helicopter where he had surgery to remove the bullet.

Yesterday he remained in Whangarei Hospital.

- Northern Advocate