CANBERRA (AFP) - Lawyers for disgraced Cardinal George Pell claimed Wednesday (March 11) he remains behind bars for child sex abuse based on "wrong" and "egregious" legal decisions, as they concluded a last-ditch appeal in Australia's top court.

The 78-year-old former Vatican treasurer is trying to overturn a six-year sentence for sexually assaulting two choirboys in the 1990s.

Pell, who once helped elect popes, is the highest-ranking Catholic Church official to be convicted of child sex crimes.

He was not present for the two-day hearing at the High Court in Canberra, but supporters and protesters gathered outside, waving Australian flags and carrying rival signs that read "keep the faith Cardinal Pell" and "Burn in hell Pell".

The court's seven judges could decide to reject the appeal - leaving Pell to serve the remainder of his sentence - or allow it, giving him the prospect of walking free. They could also send the case back to a lower court to reconsider.

A jury found him guilty in December 2018 on five counts of abusing the 13-year-old choirboys at a Melbourne cathedral when he was archbishop of the city.

That verdict was upheld in August 2019 by a three-judge panel in a 2-1 decision.

Pell's barrister Bret Walker told the High Court Wednesday there were "compounding improbabilities" in the case that had pitted the powerful clergyman against a former choirboy victim now in his 30s.

Mr Walker said the "shocking allegations" of sexual abuse were said to have taken place in a crowded cathedral and another public space, which he argued was "quite different" from typical sexual offending that tended to occur in "covert" settings.

Mr Walker also argued the lower court judges took a "terribly damaging, wrong route", effectively required Pell to establish his innocence, rather than simply raise doubt about the case against him, a decision he described as "egregious".

"That amounts to a reversal of onus and, most importantly, a misapplication of the essential protection provided by the requirement of proof beyond reasonable doubt," Mr Walker said.

The prosecution has not yet presented its response, but in written submissions to the court called the appeal "problematic", saying the defence argument "glosses over evidence" that supports the victim's account.

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

A full bench of seven judges, including Chief Justice Susan Kiefel, is hearing the appeal.

They could quash the appeal immediately, wait several months to hand down their ruling, or even send the case back to a lower court to reconsider.

The high-profile case is being closely watched by Australia's legal fraternity, the public and the world's media, with a large queue forming outside the courtroom before the building opened on Wednesday.

Pell has been removed from top Church bodies by the Vatican, but remains in the priesthood.

The Vatican previously said it would avoid launching an investigation into his conduct until after all legal avenues are exhausted.

The father of the dead victim said he was eager to avoid dragging out an already lengthy process even further.