A Western Australian policeman who tasered a driver during a scuffle following a random breath test has been found guilty of two counts of assault, a conviction the police union said would cause ramifications “across every police jurisdiction in Australia”.

Senior Constable Grantley James Keenan tasered James Bartlett in Fremantle in March 2017 and much of the incident was filmed by a rear-seat passenger.

Keenan, who has 25 years’ experience as an officer, was convicted on Wednesday in Perth magistrates court.

Magistrate Elaine Campione found Keenan was not acting in self-defence, rather 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 Corruption and Crime Commission last year also formed an opinion of serious misconduct.

WA police union president Harry Arnott said the union was disappointed with the outcome and would continue to stand by 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.

Bartlett’s breath test was negative but he had a defect notice placed on his Jeep for a noncompliant light bar on the roof and oversized wheels.

Someone in the vehicle allegedly told a junior officer they were too poor to afford the car, which prompted Keenan to approach and “have words”.

Keenan testified that Bartlett flashed the light bar, which caused “temporary flash blindness”.

As Bartlett started to drive off, Keenan stopped the car, claiming he was concerned for his colleague’s safety.

Keenan said he opened the driver’s side door and reached in to remove the keys but Bartlett hindered him.

Keenan claimed he feared he would be dragged under the “monster” tyres if the vehicle started to move again, so he warned Bartlett he was about to be tasered before deploying the device.

He then pushed Bartlett against the car during his arrest.

Campione relied significantly on the video and found Bartlett was truthful, while Keenan was not a credible witness.

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

Keenan, who was stood aside from operational duties after he was charged, will face a sentencing hearing on 10 June.

Prosecutors may argue for a suspended jail term, while Keenan’s lawyer indicated he would urge the magistrate to issue a fine.