A young New Zealand man with a metal rod embedded in his head has calmly driven to a service station and walked inside to ask for help.

The 23-year-old is believed to have been assaulted in Katikati, two hours south-east of Auckland, late last night.

Security footage from the Katikati Caltex shows the gory scene that shocked others in the service station and left owner Lester Gray believing he was being pranked.

The long steel rod was a tyre iron that was lodged in his temple.

"I was like, 'Man is this a TV show? Is this guy for real?" Mr Gray told 3News .

"It looked like a Halloween stunt where people where those Halloween costumes over their head.

"But it was pretty real when he turned around and blood was coming down the side of his head."

Dylan Shea, who was at the service station buying a pie with friends, said the victim was calm and even asked for the attendant to phone police instead of an ambulance.

"He just walked in with the rod in his head and I was just like, 'Is that a drumstick in your head?' And he was like, 'Nah, it's a rod'," Mr Shea told 3News.

Police are investigating the assault and have reportedly spoken to witnesses.