SA mum sexually abused daughter

An Adelaide mother has been jailed for performing sex acts on her young daughter and sending photos of the crime to her teacher boyfriend whom she wanted to keep "at all costs".

She also had sex with him in front of her 10-year-old daughter, after photographing him repeatedly raping the girl.

"You were her mother, for goodness sake," said Judge Simon Stretton while sentencing the pair on Friday in the South Australian District Court.

"This was very serious offending against a defenceless child by the very people who should have been protecting, rather than offending against her."

He jailed the woman, 35, and the man, 37, for five years and six months, with a non-parole period of three years and six months.

The man pleaded guilty to four counts of unlawful sexual intercourse and to aggravated counts of producing and possessing child pornography.

The mother admitted five counts of unlawful sexual intercourse, an aggravated count of indecent assault and two aggravated counts of producing child pornography.

The crimes came to light when police found the photos while investigating the teacher over online conversations when he allegedly purported to be a child.

"You were plainly both willing participants in the aberrant sexual activity performed in this matter," the judge said.

Psychologists described their mutual thinking as "a series of cognitive distortions" formed over time to justify their actions.

The woman had met the teacher, who wanted to "push the boundaries of sexual behaviour", shortly after her relationship with the father of her three children ended acrimoniously.

"He was good to you and you became determined to please him and for the relationship to survive at all costs," the judge said.

The offending had a severe impact on the victim who continued to suffer serious trauma.