A 45-year-old man accused of abusing young children during organised "swinging" group sex parties in Perth has been charged with a further 79 offences against five children, including his two daughters and two step-children.

The man was originally charged last month after a child sex investigation by WA Police, who obtained footage allegedly showing an eight-year-old girl being abused by her mother, stepfather and a male friend of the pair.

Police now allege between 2001 and 2006, the man sexually assaulted his two daughters, who were both four years old at the time of the first alleged offence.

He is facing four charges in relation to those allegations — two counts of sexual relationship with a child and two counts of indecently dealing with a child under 13 years.

"It will be further alleged between 2011 and 2015, the 45-year-old man sexually assaulted his stepdaughter and stepson who were aged between five and six at the time of the first offence," a WA Police spokesman said.

"He has been charged with 65 offences relating to his stepchildren."

The charges include 41 counts of indecent dealings with a child, 14 counts of sexual penetration of a child, four counts of stupefying in order to commit an indictable offence and three counts of encouraging child sexual behaviour.

The man is further alleged to have sexually assaulted a 14-year-old girl in 2011.

Three people were charged and three more arrested after the child sex abuse investigation. ( Supplied: WA Police )

A second man previously charged by Taskforce Mirzam detectives has also been accused of an additional 22 offences.

"It will be further alleged between 2011 and 2015, a 56-year-old man sexually assaulted a boy and girl who were aged five and six at the time of the first offence," the police spokesman said

The offences include 15 counts of indecent dealing of a child under 13 years, five counts of sexual penetration of a child under 13 years, one count of persistent sexual conduct and one count of stupefying in order to commit an indictable offence.

Sorry, this video has expired Perth Police launch major investigation into suspected sexual abuse of young children

Mother remanded in custody after court appearance

A woman charged with more than 90 child sex offences as part of the police investigation was remanded in custody after appearing in court more than a week ago.

She is accused of drugging and sexually abusing her daughter, her son and a 14-year-old girl.

Sorry, this video has expired Mother facing more than 90 child sex charges taken into custody

Police allege at least some of the abuse took place during organised "swinging" parties staged to allow group sex activities in Perth.

Detectives have been examining a number of hard-drives, laptops, thumb drives and mobile phones that were seized in raids on six properties in Perth and one in the Pilbara region late last month.

A firearm, ammunition, drugs and drug paraphernalia were also seized.

Police said last month they were investigating allegations the eight-year-old girl and at least two other victims were given methylamphetamine or cannabis at the time or just prior to the sexual abuse.

Sex Crime Division Superintendent Sharon Leonhardt said at the time that police were seeking public help.

"Specifically, we believe there are people who engage in organised swinging sexual activities who may have heard about or even been unwittingly exposed to activities of these children," she said.

"Consensual sex, group sex between consenting adults is not a crime and this is all about protecting our most vulnerable in the community, so I would say they can give us this information anonymously, they can ring Crime Stoppers."

The two men are due to appear in Perth Magistrates Court on Wednesday.