Mulroy was jailed in May for two years by a court in northwest Siem Reap but he was acquitted of the more serious charge of procuring child prostitution after an independent report cast doubts over the case and alleged a conspiracy to extort money.

Following its release, Action Pour Les Enfants (APLE), an NGO which made the initial charge, withdrew legal representation and Colonel Chea Heng of the Anti-Human Trafficking and Juvenile Protection Division in Siem Reap failed to attend any of the hearings.

So Mosseny, from Mulroy's defence, appealed the decision after the court, prosecutors and police failed to say which of the six boys had been assaulted and how, but he added that a full acquittal would be difficult.

"Given the evidence presented in court he should be acquitted of everything but my hope is to at least maintain the status quo given the position of the prosecution," he told AAP.

Prosecutors have also appealed, arguing charges of child prostitution should stand and that the sentence handed down for indecent assault was too lenient.

"When I heard the position of the prosecutors, that he should be found guilty of all charges, I knew it will be difficult to acquit Mr Mulroy," So Mosseny said.

With time already served taken into account, the former Catholic teacher from Lismore would have been eligible for release next July.

If Mulroy's appeal fails and a conviction of engaging children for sex is obtained then he faces up to 15 years behind bars.

All six children and their parents were in court. The Australian embassy in Phnom Penh also sent an observer to the hearing which is being held in the provincial city of Battambang.

So Mosseny said the judges in the appeal hearing had given ample time for both sides to present their arguments although he had been cut short during questioning.

"Today was practically a retrial. We had to start everything from scratch, again," he said.

Mulroy's case has generated widespread interest after copies of the report were sent to Australian Prime Minister Scott Morrison and other cabinet ministers.

He had initially worked for an orphanage but left after disagreements over finances and treatment of the children. He then established his own school, Education House, amid petty rivalries among foreign run NGOS in Siem Reap.

The report also said police interviews with the boys were not conducted with any adult supervision and made "under extreme duress, intimidation and extortion" in order to secure a conviction against Mulroy.

A decision on the appeal will be delivered on September 29.

Australian Associated Press