Brodie James Alderson, 21, represented himself in Tamworth Local Court earlier this week when he was convicted and fined $250 for the single larceny charge.

Magistrate Julie Soars warned the young man to be careful in his interactions with police because things can escalate quickly.

"Especially during these difficult times ... police are out there on the frontline and I think this is a bit of a prank gone wrong, is it?" she said.

"Yeah," Alderson answered after dialling into court on the phone.

"Reflect on your behaviour and I hope not to see you back before the court," Ms Soars said.

Police facts show Alderson admitted to picking the mirror up off the ground and running away with it to protect a friend, who had punched it off the police car out of "frustration" while it was parked on Peel Street on the evening of August 8.

Police facts show officers were responding to a report at a hotel in the area at the time and had asked a group of people to move on after they made "oinking sounds" towards police and acted in a "quarrelsome" manner.

Police stayed inside the hotel for about 20 minutes, but when they came out they noticed the side mirror on their marked car was missing.

Police claimed the vehicle was unable to be used for first response duties because the mirror was missing and the car was unroadworthy.

Investigations led police to track down men who then attended the Tamworth station later that night and told officers what had unfolded.

Alderson brought the stolen mirror with him and told police he was unsure what he had planned to do with it.

But Alderson, who told the court he worked as a fitter, and his "mate" who he said worked as a panel beater, then fixed the mirror back onto the car for police without damaging it.