Girls abused, filmed by defacto dad

A man who systematically sexually abused his defacto partner's two young daughters and recorded his crimes on his phone has been jailed for 12 years.

The 23 year-old, who cannot be named for legal reasons, pleaded guilty to the abuse of the girls, aged four and seven at the time, after police were informed by his defacto partner.

But WA's District Court was told that her call to police was only made months after the she had discovered the abuse, and was in part motivated by finding out that the man had cheated on her with a 15-year old neighbour.

He was also convicted of one count of sexually abusing the teenager.

The offences happened at an Armadale house between November 2011 and November 2012, when the man had been made unemployed and began heavily abusing alcohol and ice.

The court was told depression, boredom and heightened sexual urges made him turn to child pornography, which then developed into sexual abuse of the sisters.

Police discovered 34 recordings on the man's phone, both videos and photographs, cataloguing the abuse of the younger girl, which all took place at home.

The court was told that in mid-to-late 2013, the man's partner discovered the recordings, which had been uploaded to his laptop and other electronic devices.

But Judge Troy Sweeney said the woman lacked "the emotional strength to end the relationship" and so did not make an immediate report to the police.

In January or February 2014, the woman's two daughters gave her a letter outlining the abuse - but still she did not end the relationship.

But in late May 2014, the mother of the two children became suspicious that the man was having a relationship with her teenage neighbour, and only then approached police about the abuse.

The court was told the mother has been charged and pleaded guilty to two charges of failing to ensure the safety of the children, and will be sentenced later this year.

Judge Sweeney described the environment in the house as "toxic", and that the man's sexual offending against the girls was close to the worst category.

"It will be miraculous if (the girls) are not affected in the long term," the judge said.

The man was jailed for a total of 12 years, but will be eligible for parole after ten years.