A paedophile couple who drugged and abused their young children, sometimes during swingers' sex sessions that were recorded, have been handed the longest jail sentences ever imposed in WA for child sex offences.

Key points: The abuse began when the girl was five and the boy was three years old

The abuse began when the girl was five and the boy was three years old The mother and her partner recorded the abuse of the children

The mother and her partner recorded the abuse of the children It came to light with the discovery of a memory card with the recordings

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

The 41-year-old mother of the children was given a sentence of 28 years, while her 47-year-old partner was given 26 years in prison.

The couple, who cannot be named to protect the identity of their victims, pleaded guilty to a string of offences, some of which cannot be reported at this time.

The District Court was told the couple, who were members of the swingers' scene, were in a de facto relationship, but the woman's two young children regarded the man as their father and called him "dad".

The woman was also a prostitute and worked from the family home.

She will have to serve 26 years and he must serve 24 years before they will be eligible for release on parole.

'Depraved' recordings saved children from abusers

The couple's crimes came to light in 2018 when a member of the public found a memory card containing two recordings in a short-term rental property and handed it in to police.

The recordings, which were previously described by a judge as "depravity of the highest order", showed the couple and other adult men repeatedly sexually abusing the woman's daughter, who was then aged about eight.

The discovery led to the girl disclosing to police that she and her younger brother had been subjected to repeated abuse by their parents.

She was also forced to take part in sex sessions with other adults, including two separate occasions involving women she said were named "Claire" and "Jody".

The sessions usually started with the watching of pornographic movies and sometimes involved the girl and her brother being given a "stupefying substance", which it was believed was methylamphetamine.

Abuse began after a nightmare

The District Court was told the abuse started when the girl was aged about five and her brother three, and lasted for about four years.

The girl, who is now in her teens and watched Tuesday's proceedings via video link in another location, said the first time it happened was after she had a nightmare and went to her parents' bedroom seeking comfort.

Another occasion was on her seventh birthday, when the court was told the man said to her he "had a surprise" for her.

She was then abused by him, her mother and two other adult males.

WA Police officers executed search warrants at six properties in Perth and regional WA as part of the investigation. ( Supplied: WA Police )

As well as abusing his two stepchildren, the man admitted repeatedly molesting the two daughters he had from a previous relationship.

That abuse also happened when those girls were aged between four and seven.

'You stole their innocence'

Judge Bruce Goetze described the couple's crimes as "horrendous", "depraved" and "perverted", saying the couple had shown no regard "whatsoever" for their victims.

He said the recording of the then eight-year-old girl indicated she was familiar with what was happening.

"[The girl] is seen to be completely submissive. She was completely aware of what was going to happen and completely submissive," he said.

"She was wholly corrupted into accepting this horrendous conduct as being normal. It is not. It is depraved and perverted."

Judge Goezte said the crimes were an enormous breach of trust, as the couple had allowed other adults to offend against the children when, as their parents, they should have been protecting and caring for them.

Victim impact statements were provided by the man's two biological daughters, one of whom said she felt "guilty" because, if the crimes against her had been dealt with, the man may not have gone on to commit the offences against his stepchildren.

But the judge said the victims should feel no shame nor guilt.

"They deserve our sympathy and respect for having emerged from the offending," he said.

"They can hold their heads up high and be proud of themselves for their achievements."

A police task force was formed to investigate the child sex abuse ring. ( Supplied: WA Police )

Addressing the couple directly, he told them: "You stole their innocence. They should not feel they have done anything wrong."

The man's two daughters were in court for the sentencing.

Outside court they said they were pleased with the sentence and relieved by what Judge Goetze said.

"Obviously me and my sister feel a bit guilty that everything happened after us," one of the sisters, who cannot be identified, said.

"He [the judge] was just making sure we would be all right about that and not to let that get in the way of anything."

The woman, who is now her 20s, said she believed she could now get on with her life.

"I just really hope the other kids can move on as well and have a happy life," she said.

WA's worst child abuse sentences

The sentences are the highest ever imposed in WA for child sex crimes.

Last year, one of the men recorded abusing the girl in this case was sentenced to 21 years in jail after pleading guilty to a string of offences, but his term was cut on appeal to 17 years.

In 2016, the man at the centre of the so called "evil eight" paedophile ring, who pimped his daughter out for sex to at least seven strangers, was given a jail term of 23-and-a-half years.

There was also a jail term of longer than 20 years given to a man who sexually abused eight children while he ran the Warminda Hostel for wards of the state between 1978 and 1983.

Two men who abducted and sexually abused a teenage boy in 2005 received terms of 20-and-a-half years and 23 years respectively.