Paul Medulla, 35, had been charged with common assault after an investigation by the NSW Police Professional Standards Command into the alleged injury of the teen during an arrest in Sydney's west in August 2022.

"It's my view that the action of Constable Medulla... was neither necessary nor appropriate in the circumstance," Magistrate David Price said in his judgment.

The prosecution had claimed Medulla was angry in the "chaotic scene" of the arrest, despite the teenager being "clearly subdued" by Medulla and his colleagues.

Medulla said it was "quite urgent" the teen was arrested following a pursuit involving police helicopters and riot squad officers that ended in the early hours of August 14, 2022.

"We're not dealing with someone arresting Mother Teresa, as it were," defence lawyer Paul McGirr said in justifying why his client used force described as 'reasonable' to subdue the teenager.

"Is stepping on someone's head an appropriate way to deal with that?" crown prosecutor Shaun Croner asked the defendant during earlier cross-examination in Parramatta Local Court on Tuesday.

"During this situation, yes," Medulla said.

A video of the incident was played in Parramatta Local Court where Magistrate David Price found the officer guilty of common assault and sentenced him to a 12-month conditional release order with a criminal conviction attached.

Mr Medulla’s employment is now under review and his lawyer, Paul McGirr, said the decision was being appealed.

“I don’t know what community would be calling for a decorated police officer, who was attempting to take a dangerous criminal off the street, to be convicted of a criminal offence,” Mr McGirr said.

“This result fails the pub test from Glebe to Brewarrina and every pub in between.”

Medulla was found not guilty of a second charge of assault occasioning bodily harm in relation to the same incident.

Two other charges in relation to a separate incident on July 22 of 2022, where the prosecution alleged Medulla assaulted an offender under arrest by kicking him in the head, were dismissed by the magistrate.