A police officer who slapped the testicles of a senior colleague as a prank has been found not guilty of assaulting him.

Key points: What started as a prank between two police officers has finished in court

What started as a prank between two police officers has finished in court Constable found not guilty of assault after "sack whacking" a colleague

Constable found not guilty of assault after "sack whacking" a colleague A "sack whack" refers to a flick or slap of the testicles

Constable Paul Michael Vella, 27, was accused of hurting leading Senior Constable Gary Stoddart, 41, so badly that he eventually had to have a testicle removed.

Constable Vella was charged with assault occasioning actual bodily harm, reckless grievous bodily harm and common assault over the prank, which took place at Glebe Police Station in May 2015.

In court, Constable Vella claimed the pair would regularly flick each other's groin regions as a joke, however Senior Constable Stoddart's version was that on this occasion his colleague grabbed his testicles and squeezed hard.

Senior Constable Stoddart said he experienced severe pain and after visiting several doctors, was diagnosed with a condition known epididymitis, where the tubes inside the testicle become inflamed.

There were no witnesses or CCTV footage, but the trial was shown medical records, text messages between the two men, and statements from their colleagues.

'Sack whack' happened up to 50 times

Magistrate Robert Williams said there was "little if any consistency" between the two work mates' versions of events, but that he did find both policemen's accounts to be "reasonable and plausible".

"However, I am unable to find beyond a reasonable doubt that the defendant is guilty," Magistrate Williams said.

Magistrate Williams said the incident was known between the pair as a "sack whack" and had happened up to 50 times previously.

Outside court Constable Vella was asked if he regretted what happened the day his colleague was hurt.

"No, not at all," he replied.

Constable Vella's barrister Danny Eid said the magistrate had made the right decision.

"Police have got a very stressful job and they play practical jokes on each other from time to time," Mr Eid said.

"But at the end of the day this case is a clear example of those days are gone aren't they."

'Surprising this sort of thing still happens'

University of New South Wales law professor David Dixon, who specialises in policing, told ABC News it was "surprising to hear that that sort of thing is still going on in the police force".

"Police departments used to be places with a very traditional male, working-class, drinking-on-the-job culture and that sort of stuff might have gone on," Professor Dixon said.

"But there has been a big change since the days of the Wood Royal Commission and from the introduction of more women in the force.

The Wood Royal Commission was held from 1995 to 1997 and, in part, examined the NSW Police Force's culture.

"Most people would think that with the reforms that have gone on in policing, that people would have grown up a bit and would now behave in a more responsible way," he said.