Two former WA Police officers have been fined after pleading guilty to assaulting a man they were arresting in Fremantle.

Key points: The assault in Fremantle's High Street Mall was captured on security footage

The assault in Fremantle's High Street Mall was captured on security footage The victim was punched seven times by one officer and left with a dislocated finger

The victim was punched seven times by one officer and left with a dislocated finger Ex-sergeant Nathan Trenberth says they were "light" punches and "of little force"

Nathan Robert Trenberth and Julian Donohoe were charged after a Corruption and Crime Commission (CCC) investigation resulted in findings of misconduct against the two.

The CCC also released security footage of the incident in the High Street Mall in September 2017, which showed a struggle developing after the victim, who was drunk and being difficult, tried to light a cigarette.

The watchdog's report said Trenberth repeatedly punched the man, identified by the CCC as Mr Wells, in the head while he was on his back on the ground.

Trenberth and Donohoe had been due to face a two-day trial in Fremantle Magistrates Court on Monday, but changed their pleas to guilty.

The court heard Trenberth, who was then a sergeant, punched the man seven times and while the first three blows were reasonable, the other four were excessive.

Former sergeant Nathan Trenberth punched the man repeatedly as he was on the ground. ( Supplied: Corruption and Crime Commission )

Donohoe admitted bending the man's finger back and dislocating it while he was handcuffed on the ground.

Donohoe pleaded guilty to a charge of assault occasioning bodily harm and was fined $3,500.

Trenberth admitted a count of common assault and was given an $1,800 penalty.

Ex-sergeant convicted of 2006 assault

The court was told Trenberth had a previous conviction for assaulting a man during an arrest at the 2006 Australia Day fireworks display.

Trenberth said he punched the man "as many times as it took to stop him kicking". ( ABC News: Joanna Menagh )

A senior constable at the time, he punched a 20-year-old man four times in the head.

He was found guilty and fined $1,500.

Trenberth was stood aside from operational duties, but returned to work after the then-police commissioner Karl O'Callaghan decided not to sack him.

He and Donohoe both resigned from the police force after the 2017 incident in Fremantle.

'They were just light punches': Trenberth

Speaking after the hearing, Trenberth said he had been confronted with an "extremely difficult situation".

"I was kicked five times and another officer was kicked in the head and Donohoe was kicked also, and that was before any punches were thrown," he said.

"I delivered the punches, there was no injury alleged, there was no injury to the male. They were distractionary techniques of little force.

"I was 102 kilos at the time. If I was punching down on a person like that he would have injuries. They were just light punches to get him to stop kicking."

"In my opinion I punched as many times as it took to stop him kicking. Not a lot of punches were connecting or they were very light, and you can see that in the footage. Unfortunately I accept obviously the plea that there were four of them that were not warranted.

The assault took place after the victim, who was drunk and being difficult, tried to light a cigarette. ( Supplied: Corruption and Crime Commission )

Trenberth also said he would not suggest to anybody that they seek a career in the police force.

"I don't want to discourage the good men and women that are currently serving — they are exceptional people — but it's certainly not a career I'd suggest to my kids, no way," he said.

"These actions happen in the course of a moment, and it's extremely difficult. Do we want officers second guessing? Because second guessing is going to get you hurt and your partner hurt and other innocent people."

"We want other officers to be acting appropriately within the confines of the law, but they have to act with confidence, and if that confidence is taken away and they need to second guess, people will get hurt."