A policeman's expletive-laden tirade at a teenage driver is under investigation by the independent police watchdog after being caught on tape.

The officer called the 16-year-old a "dickhead" and "idiot" and repeatedly used the f-word during the stop, apparently because he believed the boy had pulled the finger at him.

He was so close during the outburst some of his spittle landed on the young man's face.

"I thought he was going to hit me," said Dyllan Vaughan, the driver.

"Every time I see a police officer now I feel nervous."

Vaughan, an apprentice butcher from Foxton, was driving home from his work in Feilding on September 3 when the officer put on his lights and pulled him over in North St. Unbeknown to Vaughan, his iPhone was on and recording. He said he'd left it on after taping a song from the radio earlier. The audio captures the exchange between the two, which begins with the officer demanding to see Vaughan's licence and asking why he thinks he's been pulled over.

"I don't know officer," Vaughan says.

The officer replies: "Yes you f.....g do. Why'd I f.....g pull you over, dickhead?"

When Vaughan tells the officer to watch his language, the policeman then calls him an idiot, and accuses him of gesturing towards him.

"I wouldn't do that," Vaughan says.

"Bullshit," the officer says. "I catch you doing that sort of bullshit again and I'll go over your car and I'll find as much trouble as I can for you."

Vaughan said the officer then threw the licence back at him and sped off. He denies giving the cop the middle finger.

"I don't know what his problem was. But I felt really intimidated. When someone comes to the window and starts abusing you and they're supposed to be a respected member of the public, it doesn't feel great," he said.

Vaughan's father, Peter Vaughan, said he thought the officer's behaviour was disgusting.

"He's a 16-year-old boy who's just got his licence," he said. "If you or I did it, it would be called gross misconduct."

The family have since made a complaint to the Independent Police Conduct Authority, which confirmed it was investigating. The family did not yet know the officer's name.

Auckland barrister Nicholas Taylor, who has also heard the tape and gave some advice to the family, said no matter what police thought Vaughn had done, he should not have been treated that way. "How can the police expect a young person to show police respect if they're going to behave like that?"

Taylor said he found it particularly concerning that the officer had made a threat against Vaughn, saying if he pulled him over again he would "go over his car and find trouble" for him.

"That brings up grave concerns for me. I find this sort of behaviour from a member of the New Zealand Police very disappointing."

A police spokesman confirmed the complaint had been passed to IPCA. "We take all complaints seriously," she said.

Police had no further comment.