A Perth police officer has been fined $2,000 and granted a spent conviction order after being found guilty of assaulting a teenager inside the Northbridge police complex.

Senior Constable Steven Trewin was found to have thrown Cassandra Swann to the ground in the early hours of August 25 2013, causing swelling to her face and breaking her thumb.

Ms Swann, who was 19 at the time, was drunk and had gone to the police station to report that she had been assaulted after leaving a nearby nightclub.

She spent about half an hour inside the station and was abusive to Constable Trewin, who claimed Ms Swann was injured as he was trying to arrest her for disorderly conduct.

However, Magistrate Andrew Maughan found that Constable Trewin was not trying to arrest Ms Swann and instead had been trying to get her to leave the station.

When he took her arm, she struck him twice and he then "threw" her to the ground in what Magistrate Maughan found was "not a reasonable response".

The assault was recorded by CCTV cameras and Magistrate Maughan said the vision "speaks for itself".

"There is no doubt Ms Swann's conduct and her assault on Constable Trewin ... were inexcusable," he said

"But equally, [Constable Trewin's] response was disproportionate and not reasonable.

"The force used was excessive."

Officer's defence went too far: magistrate

In deciding a fine was appropriate, Magistrate Maughan described Constable Trewin as "generally a conscientious police officer".

"[Ms Swann] put you in a position where you were entitled to defend yourself. I simply found you took that defence too far," he said.

Constable Trewin was granted a spent conviction order, meaning he will not have a criminal record.

Outside the court, Police Union president George Tilbury said the union was disappointed with the decision.

"We believe it sends the wrong message to the community and police officers who have to make decisions in dynamic situations," he said.

"This lady assaulted Senior Constable Trewin twice, and the magistrate has determined that the force used was excessive.

"We dispute that, and we intend to appeal against the decision."

Ms Swann, who is now 21, was in court for the decision and outside the court she apologised to Constable Trewin.

"I'd like to apologise to Officer Trewin for my actions that evening, but I do think that things could have been handled a little bit differently," she told journalists.

"No man or woman deserves, as I would say, to be rag-dolled to the floor like that."