A Timaru police officer has escaped conviction for repeatedly punching a handcuffed youth in the back of a patrol car.

Constable Marcus John Dominey was spat at and taunted and "clearly provoked" when the assault occurred in June 2014.

He was found guilty of one charge of common assault.

Judge Raoul Neave has found that the circumstances of the case, and the handicap a conviction would be while looking for a new job, made him eligible for a discharge without conviction.

The 38-year-old's lawyer Pip Hall said it's likely Dominey will be be kicked out of the police force.

He's been ordered to donate $500 to a local Women's Refuge.