A police officer has been fined over a "payback" assault on a 16-year-old girl in a cell at the Perth watch house where he pushed her, sat on her and tried to hit her.

Key points: Andrew Barber assaulted the girl after she kicked a door and it struck him in the face

Andrew Barber assaulted the girl after she kicked a door and it struck him in the face The magistrate said his actions were a gross overreaction simply to punish the girl

The magistrate said his actions were a gross overreaction simply to punish the girl Barber was fined $2,500 but will not have a conviction recorded against his name

WA Police Senior Constable Andrew David Barber was immediately stood aside from operational duties after the assault, which took place on the night of May 25.

It was recorded by CCTV cameras in the watch house building.

Barber, 36, pleaded guilty at his very first court appearance, with the Perth Magistrates Court told the teenager had been arrested in William Street in the city for attempted robbery.

She was taken back to the watch house and placed in a holding cell, the door of which Barber decided to leave open so she could get some fresh air.

Senior Constable Andrew Barber pushed the girl onto a concrete bench and sat on her. ( ABC News: Jon Sambell )

The court was told the girl was being abusive and trying to walk out of the cell, and when he tried to close the door she kicked it with her right foot, causing it to strike Barber on the right side of his face.

He then walked back into the cell and put his right hand on the victim's shoulder and his left hand on her head, pushing her onto a concrete bench before using his body weight to sit on her.

He also attempted to strike her, with the prosecutor saying the incident was estimated to have lasted about 7 seconds.

Barber was fined $2,500 and granted a spent conviction order, meaning a conviction will not be recorded against his name.

He was also ordered to pay more than $700 in court costs.

Use of force 'a gross overreaction'

Magistrate Greg Smith said Barber's actions were a gross overreaction and the force he used against the girl was unnecessary.

Mr Barber's lawyer argued the assault was "unlikely to be replicated in the real world". ( ABC News: Jon Sambell )

He said the assault appeared to be for no other reason than to punish the girl, whom he accepted was being abusive.

But Magistrate Smith said the offence was serious because Barber was in a position of authority and the girl was a juvenile who was in his care and custody.

He accepted the senior constable was of otherwise good character and that it was unlikely he would offend in such a way again.

Officer deeply remorseful, court told

Barber's lawyer Richard Yates said the girl's behaviour on the night oscillated between being quiet and co-operative, and being difficult, argumentative and abusive.

Prosecutors said Barber held a position of authority over the vulnerable girl. ( ABC News: Adam Haynes )

He said at the time his client was angry with the girl over the door hitting him, and with himself for deciding to leave the door open — a decision he now regretted.

Mr Yates said Barber was deeply remorseful and accepted he had made the wrong decision because he was in a position of authority over the girl.

He said Barber had been in the police force for 11 years and had a good service history.

"[The assault] occurred in unique, volatile circumstances which are unlikely to be replicated in the real world," Mr Yates said

Assault an abuse of power: prosecutor

The application for the spent conviction was opposed by prosecutor Caroline Chapman, who described the assault as "payback".

"[Barber] deliberately entered the cell to get payback for the door hitting him in the face," she told the court.

"She did kick the door but her behaviour was not uncommon and does not mitigate the assault.

"She was no threat to his safety and he could have simply shut the door."

The prosecutor also said the assault was an abuse of power because the girl had been particularly vulnerable and Barber held a position of authority over her.

'Decisions to be made' about officer's future

WA Police Minister Michelle Roberts said there was no excuse for Barber's actions.

Michelle Roberts says Andrew Barber "doesn't represent police officers in this state". ( ABC News: James Carmody )

"I commend him for pleading guilty, but I think the evidence was obviously very clear cut," she said.

"There are decisions to be made by the Commissioner of Police in terms of that officer's future, I understand he's been on desk duty since the initial charge was put in place."

Ms Roberts said it was a "single incident that shouldn't have occurred" and most police officers acted appropriately in the line of duty.

"Occasionally, someone does the wrong thing. That officer doesn't represent police officers in this state," she said.

"I am very confident the vast majority of our police officers treat people in custody appropriately and with respect."