Police in Western Australia's north are concerned about an increase in children committing sexual assaults, a trend child advocates say is being seen nationally.

In the 12 months leading up to August 2016, police investigated 18 children for sexual assault, a sharp rise from the same period in 2014, when just three were investigated.

Senior East Kimberley police detective Tania MacKenzie told the ABC she was shocked by the number of reports she was receiving in relation to children molesting other young people, and even forcing themselves on adult women.

"The amount of juveniles that are now starting to get involved in that sexualised behaviour is concerning," she said.

"We had a period last year when we were investigating more than a dozen allegations made against kids from as young as nine, up to age 17.

"A lot of the time for ... sexual penetrations without consent, there have been times which are opportunistic.

"The other thing we've found is the same opportunistic behaviour, where a group of young boys are together and they've seen a girl walking past that they want to steal something off, but you end up with groping behaviour and inappropriate touching."

Many of the cases are still before the Children's Court, and only limited details can be reported to protect the identities of both the alleged offenders and victims.

But the ABC can reveal cases recently investigated include:

An 11-year-old girl who fell pregnant after having sex with two teenagers, who have both been charged with sexual penetration of a child under 13. The girl is now aged 12, and is raising the baby with the support of family. A paternity test has been done to establish which of the 14 year olds is the father.

A 13-year-old boy arrested for indecently assaulting a 26-year-old woman in her home, then later raping a 34-year-old woman in the front yard of a property in Broome.

A 13-year-old boy arrested for indecently assaulting a 26-year-old woman in her home, then later raping a 34-year-old woman in the front yard of a property in Broome. A 11-year-old boy accused of sneaking up on a school-teacher in a carpark and groping her. Within months he was then arrested for indecently assaulting women in two separate incidents at train stations in Perth.

Detective Sergeant MacKenzie said it was a disturbing trend.

"That sort of behaviour has a huge impact on the victim, and I don't want to see an escalation in it," she said.

Sexual abuse by children on rise in Australia

Child protection specialist Karen Flanagan said children who did not come from violent homes were starting to offend. ( Supplied: Save the Children )

Karen Flanagan, a child protection advocate with Save the Children, has been studying sexual abuse amongst children for more than three decades, and said it is thought problematic behaviour is increasing in Australia.

"There's evidence that there's been a decrease in northern America and the United Kingdom, for example, in their statistics, but it seems in Australia it seems the figures are growing," Ms Flanagan said.

"We're seeing kids engaging in this behaviour that don't come from violent homes or don't come from violence communities, so it's important we try to prevent this."

A report commissioned by the Australian Crime Commission in 2010 highlighted the tensions that lie within how children who commit indecent assaults or rapes are treated — are they themselves victims or offenders?

"Children and young people with sexualised behaviours are very often children who have experienced harm of some kind, and who then go on to cause harm themselves," reporter author Wendy O'Brien wrote.

Online pornography 'increases offending'

Ms Flanagan said while there were established links between family and community violence, sexual abuse, and socio-economic and children committing abuse, there was a new and disturbing trend emerging.

"There are clear links where children who engage in sexually abusive behaviour have grown up in communities, or families and homes where there is a lot of violence, or has included their own trauma which includes sexual abuse, as well as socio-economic challenges," she says.

"Worrying though, a new and emerging trend is the exposure to pornography online, and children and young people, particularly those who live in rural and isolated place, who maybe don't have other forms of stimulation, will engage in viewing pornography which is well beyond their mental capacity to understand."

Ms Flanagan said this trend was behind an increase in offending from children who would not normally have abused other children.

Children can only be prosecuted for sexual offences if aged ten years or over. If convicted, they are usually sent to juvenile detention.

Social workers in the Kimberley who spoke to the ABC on the condition of anonymity said their main concern was that boys convicted of sexual assaults were returning to their community without any ongoing treatment or support to address the psychological issues associated with their offending.

"Putting a young person in a facility without treatment and intervention is just a recipe for disaster," Ms Flanagan said.

"They need to be given access to treatment programs because it's been proven that young people can and will change their behaviour.

"But that's the big problem, they're not very available.

"So every state and territory has something, but it varies dramatically."

In a written statement, the WA Department of Corrective Services said tailored treatment programs are in place for children in custody.

However, it confirmed its psychologists responsible for counselling young people post-release are all based in Perth, meaning they'd be required to travel 2,000 kilometres north to have an appointment with convicted child sex offenders in the Kimberley.