For Australian Colin Singer, his belief in Australia's justice system was destroyed almost a decade ago.

Key points: More than 100 children allegedly held in adult jails on people smuggling charges between 2008-12

More than 100 children allegedly held in adult jails on people smuggling charges between 2008-12 Indonesian court hearing compensation case against the Australian Government

Indonesian court hearing compensation case against the Australian Government Australian Government seeking to have the case dismissed

Volunteering as an independent prison visitor in Western Australia, Mr Singer was horrified when he discovered Indonesian children as young as 13 were incarcerated in adult prisons.

"I'm Australian, I used to believe that our judicial system was fair and transparent and it delivered justice," Mr Singer told 7.30.

"In every aspect I am appalled and continue to be appalled at the treatment of these children."

The first boy Mr Singer had come across was Ali Jasmin or Yasmin, in Perth's maximum security Hakea prison. The Indonesian had been tried and jailed as an adult.

At the time the Australian Federal Police used a controversial wrist x-ray method to determined he was 19 years old.

But Jasmin was actually 14 and spent two and a half years incarcerated in the adult jail.

"He looked even younger than 12, he was just a very young petrified child," Colin Singer recalls.

Ali Jasmin was held in Perth's Hakea adult prison when he was 14. ( ABC News )

After a seven-year fight, Jasmin is now on the verge of an unprecedented legal victory in the Western Australian criminal court of appeal where his conviction will soon be quashed.

"The Commonwealth Director of Public Prosecutions has, quite properly in my opinion, agreed that a miscarriage of justice occurred," Mr Singer said.

Australian agencies not subject to Indonesia court: Government

Jasmin was not the only victim, and his pending victory could set the stage for more action in Australian courts.

In a separate court case in Jakarta's Central District Court, lawyer Lisa Hiariej on Tuesday continued a fight for justice for another 115 Indonesian boys who were allegedly held in adult prisons and detention centres in Australia between 2008 and 2012.

The Australian Government did not send legal representation to the first court hearing in February and was not present again on Tuesday.

It is seeking to have the case dismissed.

In a letter read by Judge Ibnu Widodo to the court, the Australian Government said that while it respects Indonesia's judicial system, "as a sovereign state its agencies are not subject to jurisdiction of the court".

The matter will be back before the Central Jakarta District Court next Tuesday.

'I never knew what inside a prison was like'

Fiazal Arsyad is part of a class action against the Australian Government. ( ABC News: Ari Wu )

Ms Hiariej is seeking just over $1 million in compensation from the Australia's Department of Immigration, Commonwealth Director of Public Prosecutions, Attorney General's department and Australian Federal Police (AFP).

"I met one child who was detained in Silverwater [prison] for a year, he was mentally ill and stressed," she told 7.30.

"They said to me, 'We are not guilty, we have been detained for years, our parents do not know that we are still alive'.

"I hope that the Australian Government or the representatives are willing to come to talk about this, to compromise, that's all we hope."

Faizal Arsyad is one of the boys Ms Hirariej is representing as part of the class action.

He was promised 15 million Rupiah ($1,500) to sail from Indonesia's Surabaya port on 12 March 2011.

On board the boat were 132 passengers from Afghanistan, Iraq and Iran. They were intercepted by Australian authorities near Ashmore Reef.

An AFP wrist x-ray determined 16-year-old Faizal was 19. He was detained at the Arthur Gorrie correction centre in Brisbane, a high-security centre for adult males.

"We were scared," he said.

"I feel innocent, I was also a victim."

Jasmin said as a 13-year-old in 2009, the lure of 15 million Rupiah was too great to resist. He acted as a cook on the asylum seeker boat to Australia.

"As a kid, I was the youngest on board, I was scared," he told 7.30.

"I never knew what inside a prison was like, and once they put me in the underground and with no sunlight I was disorientated between night and day."

Children 'wilfully and knowingly imprisoned'

Volunteer prison visitor, Colin Singer says the AFP 'knowingly imprisoned' the children. ( ABC News )

Mr Singer said at the time in Western Australia alone there were up to 60 Indonesian children imprisoned in adult facilities.

"I am quite clear in my own mind that the Australian Federal Police wilfully and knowingly imprisoned these children," Mr Singer said.

In a statement, the AFP said the wrist x-rays were a legislated method available at the time.

"Subsequent research and court decisions raised concerns about the validity of wrist x-rays and the practice was discontinued," it said.

Mr Singer and Jasmin's legal team believe it is unlikely the case for compensation will succeed in the Jakarta, but the matter could be taken to an international court.