The inquest into the murder of brothel madam Shirley Finn has been told a senior detective named Arthur Simms left a group of colleagues "gobsmacked" when he allegedly confessed he was the one "who pulled the trigger".

The evidence came from former police officer, Chris Ferris, who testified the confession was made at a regular social gathering of officers in the Pinjarra station after Mr Simms, who'd previously been in the CIB, took over as the officer in charge.

Ms Finn's body was found in her Dodge sedan near the Royal Perth Golf Club in June 1975.

She had four bullet wounds to her head.

Giving evidence via telephone, Mr Ferris said Mr Simms told the "gobsmacked" group that Ms Finn "was not playing the game and he pulled the trigger on her".

"It startled me, it alarmed me. I stopped going to the gatherings because I didn't like hearing that stuff … if he was guilty, I didn't want to be around him," he said.

"Was he telling the truth ? I wouldn't know but he seemed to be adamant he'd done it."

Mr Ferris said he had been "haunted" by the confession ever since but maintained he had never been asked before to make a statement about it.

When asked if he said anything to his superiors at the time, Mr Ferris replied: "I would have been finished in the police force if I did that."

"The repercussions would have been catastrophic if you went and told someone. You would be drummed out of the job."

Ferris rejects grudge motivation

Under questioning from David Leigh, counsel for the Police Commissioner, Mr Ferris rejected suggestions that he was motivated by "resentment' or "a grudge" against Mr Simms, who he admitted had tried to get him sacked.

Mr Ferris claimed after he stopped going to the social gatherings Mr Simms had "turned on him" and "verballed him" to try to get him falsely charged with stealing,

But Mr Ferris said it ended up being only an internal departmental matter, after which he got a transfer out of Pinjarra station and went on to do 27 years of frontline policing.

"I have no animosity towards the man because the day he made the confession I treated him with great caution."

Arthur Simms is the third detective to be directly implicated in Ms Finn's murder in evidence at the inquest.

Previous witnesses have named the head of the Vice Squad, Bernie Johnson, as possibly ordering the shooting, while one witness identified former CIB chief Don Hancock as the person who killed her.

Earlier in his evidence Mr Ferris rejected claims from an "informant" that he had told someone at a private party in Tom Price that he had witnessed "the execution" by police "of a well dressed woman in Como."

Mr Ferris described the claim as "bullshit", saying he had never seen anybody executed, apart from on TV.

"That's never happened in my life. I've never seen a woman executed. I've never seen police officers execute a woman."

The inquest continues.