Jailed former Vatican treasurer George Pell will find out within weeks whether his conviction on child sex offences stands, is overturned or he has to face another trial after prosecutors on Thursday urged an Australian court to reject his appeal.

Pell was sentenced in March to six years in jail after a jury found him guilty of five sexual offences against two 13-year-old choirboys at St Patrick's Cathedral in the late 1990s, when he was archbishop of Melbourne.

The 77-year-old, the highest ranking Catholic cleric worldwide to be convicted of child sex offences, is mainly appealing on the grounds that the verdicts were unreasonable based on the evidence, including the account of one of the victims.

His lawyers also argue that the trial judge erred in blocking the defence from showing a video animation depicting where people were in the cathedral after Mass.

The claimant said the first assault took place in the priests' sacristy after Mass in late 1996.

Pell's lawyers say he could not have been in the sacristy at the time as he would have been on the front steps of the cathedral after Mass, a point that went unchallenged at the trial.

They also said there was a "fundamental irregularity" as Pell did not make his plea physically in the presence of the jury panel, but rather by a video-link.

If the court overturns the verdict, Pell will be released.

If the court finds the judge should have allowed the video animation or the arraignment was improper, Pell could face a retrial.

'COMPELLING WITNESS'

Responding to the appeal, prosecutors said the jury believed the complainant's account of what happened in two separate incidents in late 1996 and early 1997, and rejected the evidence of other key witnesses, including the priest who assisted Pell most closely at the cathedral where the incidents took place.

"The complainant was a very compelling witness. He was clearly not a liar. He was not a fantasist. He was a witness of truth," said Christopher Boyce, lawyer for the Crown, told the court on Thursday in his opening remarks.

The three appeal judges focussed their questions on why they should believe the complainant and disregard a range of evidence around church practices which suggested it would be implausible for Pell to have sexually assaulted two boys shortly after Mass in places where there might have been witnesses.

Boyce highlighted inconsistencies in the evidence of church staff and Pell's master of ceremonies and said the "visceral nature" of the victim's testimony made his account compelling.

Justice Mark Weinberg said juries "don't just look at a witness and say, his demeanour is impressive and therefore I accept what he says...You test against what other witnesses say."

While he questioned the prosecution's arguments, Weinberg said in previous judgements he had noted that "juries nearly always get it right, emphasis on the nearly always".

The judges also said there could be uncertainties in the witnesses' evidence, given that 22 years had passed since the events occurred.

"The question of uncertainty seems to me to be a feature inevitably about events decades earlier," Maxwell said.

The two-day appeal hearing ended on Thursday, with the judges planning to hand down their decision at a later date, which a legal expert said might be in four to eight weeks.

Pell was hand-picked by the Pope in 2014 to oversee the Vatican's vast finances, but no longer has any position in the Vatican.

He remains a cardinal and can only be dismissed from the priesthood if the Vatican's Congregation for the Doctrine of the Faith found him guilty following a separate canonical trial or a shortened procedure called an administrative process.