A motorist has become emotional while testifying he was never given a reason for being tasered by a police officer following a random breath test in Fremantle.

Key points: Police issued James Bartlett with a yellow sticker after he was pulled over for an RBT

Police issued James Bartlett with a yellow sticker after he was pulled over for an RBT Mr Bartlett was tasered by police after he flashed his car lights at them

Mr Bartlett was tasered by police after he flashed his car lights at them Senior Constable Grantley Keenan's lawyer accused Mr Bartlett of "being a smart arse"

Senior Constable Grantley James Keenan is on trial in the Perth Magistrates Court, accused of unlawfully tasering and pushing motorist James Bartlett after a roadside breath test on the evening of March 31, 2017.

Senior Constable Keenan has pleaded not guilty to two counts of common assault.

Mr Bartlett told the court he was driving to a restaurant for dinner with his wife and a friend when he was pulled over by police on Beach Street.

He submitted to a random breath test which recorded no alcohol in his system.

But he was issued with a vehicle defect notice, or "yellow sticker", for modifications he made to his bright green Jeep Wrangler.

'You're about to be tasered'

Mr Bartlett told the court that once the defect notice was issued, the police officer who conducted the breath test told him he was free to leave, and Mr Bartlett flashed his lights to indicate he was about to drive away.

He said Senior Constable Keenan, who had been sitting in a nearby patrol car, then ran towards him, yelling at him to stop his vehicle.

Senior Constable Keenan is facing two counts of common assault. ( ABC News: Charlotte Hamlyn )

The court heard the vehicle was stopped and Senior Constable Keenan then tried to remove Mr Bartlett's keys from the ignition.

A mobile phone video played to the court, which was taken by Mr Bartlett's rear passenger, shows Senior Constable Keenan warning Mr Bartlett: "You're about to be tasered".

Mr Bartlett said he was then tasered while sitting in the driver's seat with his seatbelt on.

"My body jolted from the shock of the weapon," he said.

Mr Bartlett then got out of the car and was pushed against it by Senior Constable Keenan, who handcuffed him and told him he was under arrest for failing to follow the directives of a police officer.

Motorist denies flashing LED lights

Senior Constable Keenan has admitted tasering and pushing Mr Bartlett but denied his actions were unlawful.

James Bartlett was accused in court of using his vehicle as "a tool of intimidation". ( ABC News )

Under cross-examination by defence lawyer Terry Dobson, Mr Bartlett denied being annoyed at police for issuing him the defect notice.

"You drove at [one of the police officers] to give him a bit of a scare?" Mr Dobson suggested.

"I definitely did not," Mr Bartlett replied.

"You were being a bit of a smart arse?" Mr Dobson asked.

"Not at all," Mr Bartlett said.

The motorist also denied flashing his extra-bright LED lights before being tasered.

Vehicle 'a tool of intimidation': officer

Senior Constable Keenan's colleague, Constable Aleksander Stanojevic, was the officer who breath tested Mr Bartlett and issued him with the defect notice.

Constable Stanojevic told the court that before Mr Bartlett was tasered, he was being verbally abusive, had reversed his car aggressively backwards and had flashed his vehicle's high-powered headlights in the eyes of police.

Mr Bartlett was tasered while still wearing his seatbelt, the court was told. ( Dave Hunt, file photo: AAP )

He testified that Mr Bartlett was using his vehicle as "a tool of intimidation".

But Mr Bartlett's wife, Amy Bartlett, told the court the decision to taser her husband "felt like it came out of the blue".

She said she did not understand Senior Constable Keenan's motivation for reaching into the vehicle to attempt to turn off the ignition.

The charges were brought against Senior Constable Keenan after a Corruption and Crime Commission investigation concluded he had engaged in serious misconduct.

Nine witnesses are expected to give evidence at the trial, which has been set down for two days.

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