An experienced police officer who ran over a young motorcyclist during a traffic stop in Adelaide's north has told a court it was an accident caused by a "momentary distraction".

Key points: A police officer accused of running over a teen motorcyclist is described in court as "very aggressive"

A police officer accused of running over a teen motorcyclist is described in court as "very aggressive" He is accused of running over Benjamin Wormwell at Parafield Gardens last year

He is accused of running over Benjamin Wormwell at Parafield Gardens last year The officer told the court that a "momentary distraction" caused the incident

Senior Constable Matthew Lumsden, of the Road Policing Section, has pleaded not guilty to two counts of driving without due care.

He told the court that he was responding to a report of street racing when he came across the two riders in McCarthy Court, Parafield Gardens, about 8:40pm on May 21 last year.

"I formed a suspicion that the rider was trying to move his bike to possibly evade me," he said.

"The rider's hands came up briefly and his hands came off the handlebars."

He said he was half out of the vehicle when he "felt the car roll forwards".

"I believe the transmission was in park but it wasn't and then I tried to stop the vehicle. I've tried to get my left foot on the foot brake," he said.

"The engine revved so I believe my foot must have hit the accelerator. It forced my body back into the car and at that point, I had no control of the car.

"I intended to safely manage it and there was an accident. Despite all of the efforts we made, there was a momentary distraction in putting the vehicle into park."

'It was an aggressive acceleration'

The teenage motorcyclist who was allegedly struck by the police officer told the court the vehicle "went over me, spat the bike out and dragged me between the footpath and the kerb".

Motorcyclist Benjamin Wormwell, 17, told the court that he had been visiting his friend, Brodie Mewett-Sweet, on McCarthy Court at Parafield Gardens when the alleged incident occurred.

Benjamin Wormwell said he suffered serious pelvic and leg injuries. ( ABC News )

He said he had backed his motorcycle out of the driveway and was on the other side of the road when a police car accelerated quickly from a roundabout, coming to a stop about 1 metre from the front of his bike.

"It was an aggressive acceleration — it was quite fast and the engine revved quite a lot," he said.

"I instantly put my hands up and proceeded to say, 'I'm sorry, I don't know what I've done'.

"He was very angry towards me, yelling and screaming. He said, 'don't you dare f***ing move, don't get off that f***ing bike'."

Prosecutors allege that as Senior Constable Lumsden went to step out of his vehicle, he did not apply the handbrake so it started to roll forward.

"Having noticed that the car was moving, he went to compress the brake but instead hit the accelerator," Senior Sergeant Greg Merrett said.

Mr Wormwell, who is an apprentice mechanic, suffered serious pelvic and leg injuries and now has plates and screws in his pelvis, as well as a rod in his femur.

'I don't know what I've done'

Mr Mewett-Sweet told the court he did not realise the car was a marked police vehicle until it stopped "directly in front of Ben".

"I heard a voice say, 'get off your f***ing bike' and then Ben put his hands in the air," he said.

"I couldn't see where he [the officer] was but I could see through the passenger window.

"I saw a rough image of him taking one step out, popping his head out and then Ben said, 'I don't know what I've done'."

He described the police officer as "very aggressive".

Mr Mewett-Sweet said he saw his friend start to get off his bike by putting the kick-stand down.

"When he was doing that, the car started to accelerate … I saw Ben and the bike go under the front of the car," he said.

Neighbour Jake Hazeal, who witnessed the incident, also described the officer as being aggressive.

Senior Constable Lumsden will give evidence this afternoon before Magistrate Sue O'Connor retires to determine her verdict.