A fly-in, fly-out worker has been cleared of involvement of one of WA's worst child sex abuse cases, with a judge finding him not guilty of abusing a young Perth girl in "swingers" sex sessions that included her parents.

WARNING: This story contains graphic content that some readers may find upsetting

Key points: Three people have been jailed for abusing the eight-year-old girl

Three people have been jailed for abusing the eight-year-old girl The girl's parents have already received record jail terms

The girl's parents have already received record jail terms Some of the abuse occurred in John Birkby's house

John Richard Birkby, 63, was charged after a depraved recording was found by a member of the public showing the girl, who was then aged about eight, being simultaneously abused by her mother, her stepfather and Birkby's brother, Tracey Coulter.

Her mother, who was a prostitute and worked from the family home, and her stepfather received record jail terms of 28 and 26 years after pleading guilty to dozens of charges of drugging and abusing the girl from the age of five.

Coulter, who also admitted what he did, received a jail term of 21 years, which was later reduced to 17 years on appeal.

The sentencing judge described the abuse as "depraved" and "perverted", saying it was one of the worst examples of family sex abuse to come before the courts in Western Australia.

The girl's mother was sentenced to a record 28 years in jail over the abuse. ( Supplied: WA Police )

Abusers stayed at accused man's house

Birkby was not on the recording, but he was investigated after the young girl disclosed to police that he had been involved, including an occasion on her seventh birthday.

The court heard the girl's stepfather told her "he had a surprise" for her, which was a group sex session allegedly with Birkby.

Birkby denied the allegations against him and faced trial last week before a judge sitting without a jury.

Hours of interviews with the girl were shown to the court — the first time the child had been seen and heard in public — in which the then 11-year-old described "John" as having a hook because "his arm got broke off" in a fire.

Birkby has a silver hook on his left arm, the result of a motorbike accident when he was younger, the court was told.

But District Court Judge Gillian Braddock said the fact that the girl could describe Mr Birkby's hook arm carried no weight in proof of the charges of the charges against him, but was only proof that the two had met.

WA Police Taskforce officers executed warrants at six properties across WA while investigating the sex abuse ring. ( Supplied: WA Police )

In a video-recorded interview with police, Birkby, a fly-in, fly-out worker, maintained the girl and her parents had started staying at his house at the invitation of his brother, Coulter, while he was away working in Darwin.

He said when he returned home after three weeks, they were still there, but he did not like them.

"As soon as I met them I knew they weren't very nice. They were junkies and junkies to me mean disease," he told the court.

Birkby denied he had ever seen the young girl naked, and that he was into "group sex" or "swinging".

Girl not a 'truthful' witness: judge

In her decision, Judge Braddock said the prosecution had "not established beyond reasonable doubt that Mr Birkby was present or any way involved in any of the allegations he faced".

She cleared Mr Birkby of all 12 charges, saying she was unable to accept the girl as a "reliable and truthful witness as to the involvement of Mr Birkby in the alleged offending".

Judge Braddock said the girl was bright and articulate, but she had on her own admission lied on three occasions and made false accusations against unknown and fictional people.

She said she was unable to distinguish, by the girl's demeanour, between her true and false accounts.

The girl was abused by both her parents, along with a third man. ( ABC News )

Judge Braddock said while she would not speculate about why the girl might have told lies, she did note that in the series of interviews she conducted with police, the girl was praised repeatedly for remembering things.

Judge Braddock also said she had to take into account the entirety of the abuse the girl had suffered.

"A significant matter is that she has undoubtedly experienced repeatedly the type of offending that she describes at the hands of her mother, father and others …," she said.

"The possibility of [the girl] being mistaken in relation to individual instances of abuse … must be high. The behaviour was normalised by its extent and repetition."

In contrast, Judge Braddock described Mr Birkby's denials in his video-recorded interview with police as "frank and direct", and his responses to detailed allegations put to him were "firm and consistent".

Mr Birkby was released immediately after he was acquitted but made no comment as he left the court building.