Jaunay was found guilty of slapping a then 17-year-old across the face during a confrontation in Whyalla in 2013, when he was a serving officer in the SA police force.
He was given a three-month suspended sentence and placed on a one-year good behaviour bond.
At his trial, the prosecution relied heavily on an audio recording of the incident in which a magistrate accepted Jaunay told the teenager that if he wanted to tell his mother that he had “belted you one” he could “go right ahead” because any such allegations would be denied.
The magistrate also relied on photographs of facial injuries the boy suffered at the time.
But in a judgment on Friday, Justice Laura Stein found the magistrate had erred in rejecting the possibility that some other words were spoken on a part of the recording that was difficult to hear.
“On my review of the evidence, I do not consider the issues raised in relation to the audio recording are such as to be a complete obstacle to a conviction,” the judge said.
But she said inferences that could be drawn from the audio should be considered in the context of all the evidence, including an assessment of the victim as a witness.
The magistrate had found the teenager to be a poor witness, offering inconsistent statements and routinely feigning confusion.
In such circumstances, Justice Stein said it was appropriate to allow Jaunay’s appeal and return the case to the magistrates court for a new trial.