A Perth policeman denied using excessive force when he broke a teenager’s thumb and forced her to the ground while arresting her at the Northbridge police station after she attended to report being assaulted.

CCTV footage of the incident was released today by the Perth Magistrate’s Court, where Sen. Const. Steven Adrian Trewin has faced trial after pleading not guilty to a charge of assault occasioning bodily harm.

Cassandra Swann, who was 19 at the time of the incident, went to the police station in August 2013 to report a man who she said had punched her in the face after she left the Paramount Nightclub in Northbridge.

Giving evidence on Wednesday, Ms Swann admitted she had been drunk, could not remember everything that happened and may have acted aggressively and swore at Sen. Const. Trewin after becoming frustrated with the way he responded to her complaint.

After being forced to the ground, which is seen in the CCTCV footage played to the court, Ms Swann had a bruised forehead and broken thumb.

The prosecution case alleges Sen. Const. Trewin threw Ms Swann to the ground with force when she posed no threat to his safety.

Today, Sen. Const. Trewin told the court he believed he had been calm and Ms Swann had been in an agitated state.

CCTV footage shows the arrest. Picture: Supplied.

“My intention was to stop her striking me and force her to the ground,” he said after electing to give evidence in his defence.

“She was in an agitated state and continued to point and swear at me. I’ve had numerous situations as a police officer of people swearing at me, I thought I was calm.”

Magistrate Andrew Maughan reserved his decision until later this month.