A leading child protection expert has warned of a rise in the number of young children sexually abusing their peers in preschool and primary schools across Australia.

Professor Freda Briggs of the University of South Australia has told the ABC that children as young as five are displaying problem sexualised behaviour.

She says while international research shows it is happening more often, there have been no attempts to measure rates in Australia.

Professor Briggs has recently written a report about the phenomenon for Taylor and Francis Online, which publishes scholarly journals and books.

"Children of say six are sexually abusing children of five, and usually it is about replicating either what's happened to them as abused victims or that they've seen too much pornography and (they're) trying to do what they've seen the grown-ups do," she said.

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Professor Briggs is currently advising parents at an Adelaide school, where a group of six-year-old boys have been forcing five-year-olds to perform oral sex on them.

One of the mothers, who wished to remain anonymous, told the ABC: "My son told me other boys in kindy were kissing penises and they were asking him to do it too.

"They asked him, 'do you want to play Star Wars?', and then they made him do it. I went to the principal and she said, 'we don't believe you, we don't believe it's happened at school'."

A Sydney mother, who cannot be identified for legal reasons, told the ABC her eight-year-old was recently sexually abused by another eight-year-old boy in the school toilets.

"He told me that this boy had [taken] him into the toilets and pulled his pants down," she said.

"He would hold [him] down there while he was going to the toilet and then it progressed to going into the bathroom stalls, pulling [my son]'s pants down, fondling, touching, telling my son that he wanted to have sex with him."

She said when she reported it to the school, staff denied it had happened. Even when the other eight-year-old boy admitted it, the principal suggested the mother move her son to another school.

And when she reported the incident to police, they told her that because the boy was under 10, no crime had been committed.

"Nobody wants to talk about it because it's not a legal crime that these kids are committing in schools. There's nothing to do," she said.

"We just have to go away and just get on with our lives and try and cope with what's happened."

In a statement the NSW Department of Education said:

"In this matter, the classroom teacher informed the principal. Contact was subsequently made with the police and the Department of Family and Community Services to advise them of the incident. "The welfare and wellbeing of children in its care is a priority for the NSW Department of Education and Communities, which has policies and procedures to deal with situations that ensure this is the case."

Schools dismiss abuse as 'normal sexual curiosity'

Professor Briggs said many schools did not respond well to reports of children sexually abusing their peers.

"Typically schools have ignored it and dismissed it as normal sexual curiosity when it isn't," she said.

"And as a consequence they haven't reported it or when they do report it, it's often reported minimally such as it's inappropriate behaviour."

Professor Briggs says the phenomenon is increasing because children are being sexually abused by adults at younger ages, and then repeating the abuse on their classmates.

She says overexposure to internet pornography is also contributing to the problem.

Her report says it is a neglected area of research and clinical practice in Australia, and teachers and social workers are not trained in how to deal with the child perpetrators or their victims.

"Teachers are just not supported; police are not interested because the children are much too young to be arrested, so there's no crime committed," Professor Briggs said.

Abuse could become 'habitual' if not addressed

Research has shown that in 80 per cent of cases, child perpetrators are re-enacting their own experiences of sexual abuse.

"They will replicate what's happened to them because it releases some of the fear if you can play at it with other people," Professor Briggs said.

"Your problem is that if the children enjoy the power that it gives them when they do this with younger children, it's apt to continue and become habitual behaviour."

In 2010, a report by the Australian Crime Commission concluded that a culture of confusion, denial and non-disclosure had created a hidden population of children at risk.

It recommended a raft of changes to ensure children in need received therapy. But Professor Briggs said little had changed.

"We need appropriate treatment for these children. You can't wait until they've been abusing for several years and only tackle them when they're in their teens," she said.

"But initially we need to be asking the first child who is doing this who showed them how to play that game. Not in a reprimanding sort of way, but to investigate."

Do you know more? Contact investigations@abc.net.au