Perth policeman denied using excessive force when he broke a teenager’s thumb during arrest. Picture: Supplied.

A policeman who threw a teenager to the ground at Northbridge police station, leaving her with head injuries and a broken thumb, has been found guilty of assault occasioning bodily harm.

Sen. Const. Steven Adrian Trewin was given a spent conviction and fined $2000 in Perth Magistrate's Court this morning over the assault on Cassandra Swann.

Ms Swann, then 19, went to the police station to report an assault after she left Paramount Nightclub in August 2013.

Sen. Const. Trewin threw Ms Swann to the ground after she became agitated, started swearing and lashed out at him.

He argued he dealt with Ms Swann for 30 minutes then decided to arrest her because she was not going to leave.

But Magistrate Andrew Maughan said he did not believe Sen. Const. Trewin was trying to arrest the teen when he assaulted her.

Mr Maughan told the court Ms Swann hit or tried to hit the police officer twice before he forced her to the ground.

He said Ms Swann put the officer in a position where he was entitled to defend himself but he took that defence too far.

Outside court, Ms Swann said no man or woman deserved to be physically assaulted the way she had.

"I would like to apologise to officer Trewin for my actions towards him that morning but I do think that things could have been handled a little bit differently," she said.

"I also want to thank internal affairs and the DDP for all their hard work."

The incident was captured on CCTV footage, which was played to the magistrate during the trial.

Ms Swann cannot remember the assault and repeatedly broke down in court when she watched the footage for the first time.

WA Police Union president George Tilbury said the magistrate's decision sent the wrong message to the community.

"We are talking about a dynamic situation that police officers find themselves in every day where force needs to be used," he said.

"We will appeal this decision because we do not agree with it, we believe there's grounds to go to the Supreme Court."