A police officer captured on CCTV punching a man in the head seven times in Fremantle in 2017 had a previous criminal conviction for unlawfully assaulting a man during an arrest at a 2006 Australia Day fireworks display.

Key points: WA's corruption watchdog made a misconduct finding against Sergeant Trenberth over a 2017 incident

WA's corruption watchdog made a misconduct finding against Sergeant Trenberth over a 2017 incident It is now revealed the officer was convicted of assault over an arrest he made 11 years prior

It is now revealed the officer was convicted of assault over an arrest he made 11 years prior He was stood down from operational duties after that incident, but was later returned to full duties

Amateur video captured the incident in which the then 32-year-old senior constable, Nathan Robert Trenberth, punched a 20-year-old man four times in the head during Skyshow celebrations.

In sentencing, Magistrate Vicki Stewart said the use of force was unreasonable, finding the officer guilty of assault and fining him $1,500.

The officer — who was dux of his year at the police academy — was stood aside from operational duties after that incident, but returned to work after the now former police commissioner Karl O'Callaghan decided not to sack him.

Second incident a decade later

By the time of the second incident, he had risen to the level of sergeant and had 21 years of policing experience under his belt.

That incident was investigated by WA's corruption watchdog, which in a report released last week, made a finding of misconduct against Sergeant Trenberth for using excessive force in arresting a man named as "Mr Wells".

CCTV vision taken in the High Street Mall shows Mr Wells was wrestled to the ground and hit seven times in the head while he was on his back before being handcuffed.

Mr Wells declined to make a formal complaint and three charges against him — assaulting a police officer, obstructing public officers and failing to provide identification — were later discontinued.

Sergeant Trenberth told the Perth Magistrates Court in 2006, he was not aiming for the man's head and was using restraint tactics which he was taught during police training.

Sorry, this video has expired WA Police officers used excessive force in Fremantle arrest, CCC says

He gave a similar explanation to WA Police's internal investigators when they questioned him about the 2017 incident in the High Street Mall in Fremantle.

"He considered his use of force to be lawful and in line with training in empty-hand techniques," the Corruption and Crime Commission (CCC) report said.

Empty-hand techniques do not involve a weapon and can include defensive movements like blocks, strikes and punches.

Officers are told their response must be proportionate to the threat.

'The punches weren't hard'

The CCC report said Mr Wells was heavily intoxicated and had been ejected from a nearby hotel, but questioned Sergeant Trenberth's witness statement describing Mr Wells as "aggressive" and "vengeful".

"CCTV footage does not support Sergeant Trenberth's insistence that Mr Wells was displaying continued aggressive behaviour towards the officers," it said.

In his witness statement about the incident, Sergeant Trenberth said:

"I threw a few punches at [the man]. I think I hit him twice. The punches weren't hard but just numerous jabs to distract and cease his attack on us."

He also maintained to internal investigators he had used reasonable force to stop Mr Wells from kicking him.

"I think the fact that it's not a criminal investigation backs that up and this is only disciplinary, so I haven't used an excessive amount of force otherwise I'd be facing an assault charge..." he said.

Sergeant Trenberth, who has since resigned from WA Police, was stood down on June 5 last year.

A WA Police spokeswoman said his resignation was effective from July 6, but declined to comment further because his actions were still under investigation.

Charges were laid against Mr Wells, but were later discontinued. ( Supplied )

Internal investigators did 'half-arsed job'

The ABC has been unable to contact Sergeant Trenberth for comment.

But in a response to a draft version of the CCC report, Sergeant Trenberth disputed the assertion he used excessive force in the 2017 incident.

He told CCC Commissioner John McKechnie that he and police internal investigators had done "a half-arsed job" with "baseless allegations".

"Yet … you say based on the CCTV footage I 'may' have used excessive force," he wrote.

"Where is it excessive? Where are the injuries? Where's the complaint from the offender I punched?"

Sergeant Trenberth wrote that he did not deliberately attempt to inflict pain on Mr Wells.

"Use of force is disturbing," he said.

"I hated this part of the job and fortunately most times I could talk my way through nearly all situations."