A 45-year-old former WA Police sergeant has been charged with assault following an internal review into the arrest of a man in Fremantle.

Key points: Sergeant Nathan Trenberth is accused of assaulting the man during an arrest

Sergeant Nathan Trenberth is accused of assaulting the man during an arrest The CCC alleged he punched the man repeatedly in the head

The CCC alleged he punched the man repeatedly in the head A second officer has pleaded not guilty to assault occasioning bodily harm

A Corruption and Crime Commission report into the September 2017 incident made findings of misconduct against two officers earlier this year.

Vision captured on CCTV of the arrest in the High Street Mall showed a man, described in the CCC report as "Mr Wells", trying to light a cigarette while speaking with police officers.

An auxiliary officer is seen to grab the cigarette, sparking a struggle which ultimately led to Mr Wells being wrestled to the ground and handcuffed.

The report said the most senior officer present, Sergeant Nathan Trenberth, repeatedly punched Mr Wells in the head while he was on his back.

Sergeant Nathan Trenberth is accused of assaulting the man in Fremantle's High Street Mall. ( Supplied: Corruption and Crime Commission )

The CCC also alleged that after Mr Wells was handcuffed, his finger was bent back to the point where it dislocated.

The report said while Mr Wells was intoxicated and no doubt difficult, the force used to overcome him was excessive.

Mr Wells did not want to make a formal complaint but WA Police started an internal investigation into the incident, leading to two officers — Sergeant Trenberth and Constable Julian Donohoe — being stood down.

The vision shows Mr Wells trying to light a cigarette before being arrested by police. ( Supplied: Corruption and Crime Commission )

Sergeant Trenberth has since resigned from the force, while Constable Donohoe was charged in February with assault occasioning bodily harm.

Constable Donohoe pleaded not guilty to the charge in Fremantle Magistrates Court in April and will face trial in August.

The former sergeant is due to face court in June and said he would plead not guilty to the charge.

He was also critical of the CCC report, claiming it published details of an internal disciplinary investigation which could hurt his defence of the charge.

"They have now been widely reported on in the public domain and will be again now I'm charged and facing imprisonment," he said.

"This will jeopardise the court process and ensures I can have little hope of a fair trial or fair hearing in the public domain."

CCTV 'inconsistent' with sergeant's story

The CCC report was particularly critical of the "disturbing" inconsistencies between witness statements from police and the CCTV vision.

The CCC report was one of a series examining the conduct of police in Fremantle. ( ABC News: Adam Haynes )

For example, Sergeant Trenberth said Mr Wells was "aggressive" and "vengeful" towards police.

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

The report also raised concerns with the treatment of Mr Wells after he was taken back to an observation cell at Fremantle Police Station.

He was kept handcuffed for longer than an hour.

The report said Mr Wells was later charged with obstructing public officers, three counts of assaulting a public officer and one count of failing to comply with a lawful order, but his prosecution was later discontinued.

The CCC report was one of a series recently undertaken into the conduct of police in Fremantle.