Police are investigating how a hunter was shot in the leg in the Tararua Ranges.

The 27-year-old Palmerston North man received serious gun-shot wounds to his upper thigh.

He was flown to Palmerston North Hospital for surgery.

Police say a party of three hunters were travelling into the ranges on quad bikes, when one of the guns went off before midnight on Wednesday night.

At this stage, police believe it was most likely an accident.