An Indonesian man who was a child when Australia jailed him in an adult prison for people smuggling was the victim of a miscarriage of justice, a WA court has ruled.

Key points: Ali Jasmin spent almost three years in adult prison after prosecutors relied on wrist X-ray

Ali Jasmin spent almost three years in adult prison after prosecutors relied on wrist X-ray Technique used to conclude he was an adult was "inaccurate and misleading", court told

Technique used to conclude he was an adult was "inaccurate and misleading", court told Mr Jasmin would have been sent home if correctly identified as a child after arrest

The decision to set aside Ali Jasmin's conviction has the potential to set a legal precedent for dozens of other Indonesian children who were allegedly wrongfully jailed by Australia between 2008 and 2012.

In a scathing judgement, the Court of Appeal ruled the original judge's decision in accepting that Mr Jasmin was an adult at the time of his arrest was unsafe and constituted a miscarriage of justice.

Mr Jasmin — who is now aged 20 or 21 — was likely to have been only 13 when he was a crew member on the unseaworthy boat, intercepted by the Australian Navy in December 2009 with 55 Afghan citizens on board.

He spent almost three years in Perth's maximum security Hakea Prison after pleading guilty to a charge of people smuggling, before he was released in May 2012 and sent back to Indonesia.

Prosecutors had relied on a wrist X-ray taken from Mr Jasmin shortly after he was arrested.

The wrist X-ray was then analysed by a Perth radiologist, who determined that Mr Jasmin was likely an adult.

However, the Court of Appeal heard the technique used by the radiologist to determine Mr Jasmin's age was "inappropriate, inaccurate and misleading".

"In truth [the radiologist's] opinion had no acceptable basis in fact or by reference to generally accepted scientific methodology," the president of the Court of Appeal, Justice Michel Buss, said.

"[The radiologist's] opinion was unsatisfactory and unreliable."

Mr Jasmin would not have been prosecuted in Australia and would have been sent back to Indonesia if authorities had correctly identified him as a child when he was arrested.

Birth certificate not given to court

The initial judge had also taken into consideration evidence given in court by Mr Jasmin, who — through an interpreter — gave differing dates of birth, including 1990, 1993 and 1996.

Ali Jasmin returned to his family in Indonesia after his release from jail in 2012. ( The Project: Sam Clark )

But Justice Buss noted a copy of his "verified" birth certificate — which stated he was born in 1996 — was not provided to the court by prosecutors or Mr Jasmin's defence lawyers, despite the fact both parties were in possession of it.

During its hearing, the Appeal Court was also provided with untranslated school records from Indonesia, as well as an affidavit from Mr Jasmin's mother who said he was born in 1996.

Justice Buss said he was satisfied "the primary judge's conclusion that [Mr Jasmin] was at the material time of the age of 18 years is unsafe".

"On the evidence before this court, the conclusion was not reasonably open," he said.