Cardinal George Pell's last-chance appeal against his child sex abuse convictions opened in Australia's top court on Wednesday, with his fate now in the hands of the country's most senior judges.

The 78-year-old, who is serving a six-year sentence for sexually assaulting two choirboys in the 1990s, was not present for the two-day High Court hearing in Canberra, where his lawyers will mount a final bid to clear his name.

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Pell's supporters gathered outside the High Court building, waving Australian flags and carrying signs that read: "Cardinal Pell is innocent forever" and "Keep the faith Cardinal Pell".

In December 2018, a jury found the former Vatican treasurer, who once helped elect popes, guilty on five counts of abusing the 13-year-old boys at a Melbourne cathedral while he was the city's archbishop.

Pell, who served as the Vatican's treasurer under Pope Francis, is the highest-ranking Catholic Church official to be convicted of child sex crimes.

He was sentenced in March 2019 and lost a first appeal in August, a decision that saw a three-judge panel split in a 2-1 verdict.

The case pitted the cleric against a former choirboy victim now in his 30s, whom two of the judges found to be "very compelling" and someone who "was clearly not a liar, was not a fantasist and was a witness of truth".

The third judge, however, found the man's account "contained discrepancies" and there was a "significant possibility" Pell did not commit the offences.

'Erroneous judicial method'

In court submissions, Pell's legal team argued the majority judges applied an "erroneous judicial method" that required the cleric "to establish actual innocence as opposed to merely pointing to doubt".

Pell's lawyer Bret Walker told the court on Wednesday that a key factor that made Pell's alleged offences implausible was that they took place after Sunday mass in a busy area of St Patrick's Cathedral in Melbourne, not in secretive or private settings, "unlike so many appalling historical sexual misconduct cases".

In response, the prosecution called the grounds for appeal "problematic", saying the argument "glosses over evidence" that supports the victim's account.

Pell became cardinal in 2003 at the time of Pope John Paul II and was appointed treasurer of the Vatican during the time of Pope Francis [File: Osservatore Romano/Reuters]

The case relied solely on the testimony of Pell's surviving victim, as the other - who is not known ever to have spoken of the abuse - died of a drug overdose in 2014. Neither man can be identified for legal reasons.

University of Melbourne law professor Jeremy Gans said it was difficult to predict the outcome of the closely watched appeal, being heard by a full bench of seven justices.

The judges could quash the appeal immediately, or wait several months to hand down their ruling.

Drawn out process

Gans said the High Court could also decide that the appeal court judges erred by viewing video evidence from the original trial and send the case back to them.

"The only big development is they've said they're interested in whether the Court of Appeal did the right thing watching video," he told AFP news agency.

"That wasn't on the table but the High Court has now potentially put it on the table," he said.

If that did occur, it would drag out an already lengthy process even further - an outcome the father of the dead victim is eager to avoid.

"This process continues to take a toll on his physical and mental health," his lawyer Lisa Flynn said Tuesday.

"If the High Court allows George Pell to walk free from jail, our client says he will lose all faith in our legal system," she added.