A West Australian police officer has been found guilty of two counts of assault after tasering a driver following a random breath test.

Senior Constable Grantley James Keenan tasered James Bartlett in Fremantle in March 2017 in an incident filmed by a rear-seat passenger.

Mr Keenan, a 25-year veteran officer, stood trial in Perth Magistrates Court and was convicted this week.

Magistrate Elaine Campione found Mr Keenan was not acting in self-defence, and that his actions were excessive and objectively unreasonable.

“The accused was not exercising any police powers and ... he was not justified, authorised or excused by law,” she said.

The trial came after a separate Corruption and Crime Commission investigation last year formed an opinion of serious misconduct.

Mr Bartlett returned a negative breath test, but had a defect notice sticker placed on his Jeep for a non-compliant rooftop light bar and oversized wheels.

Mr Keenan testified that Mr Bartlett switched on the light bar, causing “temporary flash blindness”.

When Mr Bartlett started to drive off, Mr Keenan stopped the car.

The officer said he was concerned for his colleague's safety, and opened the driver's side door to reach in and remove the keys.

Mr Keenan said Mr Bartlett hindered his attempt to take the keys and, fearing he would be dragged under the “monster” tyres if the vehicle began moving, Mr Keenan warned Mr Bartlett he was about to be tasered.

The officer then deployed the device before pushing Mr Bartlett against the car to make his arrest.

Ms Campione found that Mr Bartlett was truthful while Mr Keenan was not a credible witness.

“His evidence is self-serving and is crafted to suit his version,” she said.

Mr Keenan will face a sentencing hearing on June 10, at which his lawyer has indicated he will urge the magistrate to issue a fine.

WA Police Union president Harry Arnott said the union will continue to stand by Mr Keenan.

“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,” he said.