A police officer who was filmed driving into an 18-year-old man in Perth's south-east last year, knocking him down and sending him to hospital, has been found guilty of dangerous driving.

Key points: Police approached William Farmer after reports he was acting suspiciously

Police approached William Farmer after reports he was acting suspiciously A police car was filmed cutting across Mr Farmer's path, knocking him down

A police car was filmed cutting across Mr Farmer's path, knocking him down He convulsed on the floor and was taken to hospital, where he was treated for bruising

Senior Sergeant Richard Stephen Moore had denied doing anything wrong when the police vehicle he was driving at slow speed hit William Farmer on Wilfred Street in Thornlie during a botched arrest.

But Magistrate Tom Hall found that while Moore had only wanted to block Mr Farmer and had not intended to hit him, his manner of driving — in turning a two-tonne car "very quickly and very sharply" — was dangerous.

In his decision, Mr Hall likened the incident to an episode of "The Benny Hill Show", but in "slow motion".

He said he accepted Moore may have been tired and frustrated that he had not been able to arrest Mr Farmer, but added that the 18-year-old was also exhausted and "not going anywhere."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 30 seconds 30 s The incident was captured on mobile phone cameras

Officer 'messed up' arrest: Magistrate

Mr Hall said the situation was not urgent and the only person who appeared to panicking was Moore, who had "totally messed up" the arrest and had "been shown up".

"He was too old, too unfit and too overweight to be doing what he was doing," he said.

Mr Hall accepted that Moore, an officer of 24 years, was of otherwise good character and was unlikely to ever offend again.

Senior Sergeant Richard Moore was fined $2,500 and had to pay $7,000 costs over the incident. ( ABC News: Hugh Sando )

He fined him $2,500 and ordered that he pay court costs of more than $7,000.

He was granted a spent conviction order, which means he will not have a criminal record and does not have to disclose the conviction to anyone.

A WA Police spokeswoman said Moore remained stood aside from duties and an internal disciplinary process was ongoing.

Passers-by catch incident on camera

The incident in May 2018 was recorded by several passers-by, with the footage showing Mr Farmer convulsing on the ground after being hit.

An ambulance was called and Mr Farmer was treated for minor injuries at Fiona Stanley Hospital.

Mr Farmer sustained bruising after being knocked down by a police car in Thornlie. ( Supplied: Mervyn Eades )

Moore's trial earlier this year heard the officer and his partner went to the area to investigate reports of a suspicious person knocking on doors and asking residents for cigarettes.

The court was told that when the officers confronted Mr Farmer, he fled and they chased him.

It also heard Mr Farmer tried to open the doors of passing cars and Moore believed he was attempting a carjacking and needed to be stopped.

At one point he managed to put a handcuff on Mr Farmer's left wrist but he broke free and continued running.

It was then that Moore drove on the wrong side of the road and cut across Mr Farmer's path, knocking him down.

Teenager 'lives in fear'

Mr Farmer's mother, Edwina, welcomed the verdict, saying her son would never be the same.

"He lives in fear every day since that day and I'm just glad it's all over," she said outside court.

Ngalla Maya Aboriginal Corporation chief executive Mervyn Eades said he was relieved the officer had been held accountable for his actions.

"The young boy has been left shattered, scared by police … he never committed no offence, he was innocent," Mr Eades said.

"We're really happy, the family's really happy that [Moore] has been found guilty."

Mervyn Eades said Moore might never had been convicted if the second video footage had not come to light. ( ABC News: Hugh Sando )

The court heard Moore was trained pursuit driver and Mr Eades said he should have known better than to use his car to try to arrest Mr Farmer.

Moore was not stood down from operational duties until four days after the incident, when an internal investigation was launched after a second video recording was made public, showing what happened from a different angle.

Mr Eades said the conviction might not have happened if not for the video footage.

"A picture says a thousand words and cannot be denied," he said.

'Considerable threat' claim refuted

In his evidence, Moore said he was concerned the handcuff he placed on Mr Farmer's wrist could be used as a weapon, but maintained he never intended to hit him with the car and was just trying to "cut him off".

His lawyer, Linda Black, submitted that at the time Mr Farmer "presented a considerable threat to the community".

She estimated Moore was only driving at six or seven kilometres per hour at the time of impact.

Senior Sergeant Richard Moore maintained Mr Farmer presented a "threat" to the community. ( ABC News: Evan Morgan Grahame )

But Mr Hall said he found some of Moore's evidence at the trial was not credible, in particular his claim that he believed Mr Farmer had to be stopped urgently because he trying to carjack vehicles.

He said that lacked plausibility because in an email chain between him and other officers on the night of the incident, Moore indicated he had not been aware of any attempted carjackings.

Mr Hall said the carjackings had been raised in the emails by his partner and it was "inappropriate" Moore had been informed of what she she had been saying when he had not yet been officially interviewed.

Other parts of Moore's evidence, such as his claims about Mr Farmer possibly using the handcuffs as a weapon, was an exaggeration and an attempt to bolster his position, the magistrate ruled.

The WA Police Union issued a statement describing the penalty as excessive.

"We are sick and tired of our members being consistently penalised to a higher degree than the average member of the community," the statement said.

"We are in no doubt that if a non-police officer faced a similar charge and circumstance, they would have received a much smaller fine."

The union added that while the officer was remorseful, it would still explore all avenues to appeal against the decision.