A Perth police officer has been found guilty of unlawfully tasering a motorist during a roadside breath test in Fremantle two years ago.

Key points: Motorist James Bartlett was tasered while he sat in his car with his seatbelt on

Motorist James Bartlett was tasered while he sat in his car with his seatbelt on Police cleared Senior Constable Grantley Keenan but a CCC probe led to charges

Police cleared Senior Constable Grantley Keenan but a CCC probe led to charges Prosecutors may push for a jail term but Keenan's lawyer says a fine is enough

Senior Constable Grantley James Keenan's conduct on the night of March 31, 2017, became the subject of a Corruption and Crime Commission (CCC) investigation, after he was cleared of any wrongdoing by three earlier internal police investigations.

The motorist, James Bartlett, had been driving to a restaurant with his wife and a friend when he was pulled over by police on Beach Street.

James Bartlett was tasered after his Jeep Wrangler was pulled over in Fremantle. ( ABC News )

A random breath test detected no alcohol in his system.

But he was issued with a vehicle defect notice for modifications he had made to his Jeep Wrangler.

The court heard once Mr Bartlett had passed the breath test he had flashed his lights, causing Senior Constable Keenan to approach his car and try to remove the keys from the ignition.

A mobile phone video taken by his passenger showed Mr Bartlett then being tasered as he sat in the driver's seat with his seatbelt on.

Keenan was charged with two counts of common assault. He admitted to tasering the man but denied it was unlawful.

But he was convicted after a two-day trial in the Perth Magistrates Court, and was released on bail until he is sentenced in June.

Magistrate did not find police officer to be 'truthful witness'

During his trial, Keenan argued his actions were necessary, claiming he feared for his life.

However in her findings, Magistrate Elaine Campione rejected his evidence, saying she "did not find the accused to be a truthful witness".

She said while Mr Bartlett was "no angel", she did find him overall to be truthful, saying that in the lead-up the assault, he had complied and cooperated with all directions without incident.

Senior Constable Grantley James Keenan was cleared by three internal police inquiries. ( ABC News: Charlotte Hamlyn )

Magistrate Campione said the "determinative factor" in the case was the video footage of the incident, which she said spoke for itself.

She found that at the time of the assaults the two officers at the scene had "exhausted their powers to require [Mr Bartlett] to stop his vehicle".

'He was free to go," she said.

"The accused was not justified in using the taser. The use of the taser constituted an unlawful assault.

"There was no imminent risk to the accused [Keenan] or to [his colleague]. He was not acting in self-defence.

"The actions of the accused are inconsistent with his evidence that he was in mortal danger.

"The prosecution has established beyond reasonable doubt that the accused was not exercising any police powers and that he was not justified, authorised or excused by law."

Prosecutors said they may argue for a jail term — although it could be suspended — while Keenan's lawyer, Terry Dobson, said he would be submitting that a fine would be the appropriate penalty.

Keenan made no comment as he left court after the verdict.

Union stands by officer

The WA Police Union will "stand by Senior Constable Keenan", according to union president Harry Arnott.

"We are obviously disappointed with this decision and we feel for our member and his family," Mr Arnott said.

"The ramifications of this decision will be felt across every police jurisdiction in Australia, and we will vigorously pursue all avenues of appeal to ensure that justice is served."

The officer was stood aside from operational duties last October.

WA Police released a statement on Wednesday, saying the Internal Affairs Unit would finalise its investigation into the officer's conduct after his sentencing and after the expiry of any appeals period.