Disgraced Catholic Cardinal George Pell's bid to overturn his child sex abuse convictions will be heard in Victoria's highest court tomorrow when his appeal hearing gets underway.

Pell, 77, is serving a six-year jail term for sexually abusing two choirboys when he was archbishop of Melbourne in the 1990s.

Last December, a jury found him guilty of sexually abusing the boys at Melbourne's St Patrick's Cathedral after a Sunday mass in 1996, and then abusing one of the boys a second time several months later.

The former senior Vatican official pleaded not guilty at trial, but was convicted of one count of sexual penetration of a child under 16 and four counts of committing an indecent act with a child.

Three Court of Appeal judges will rule on Pell's appeal after a hearing that has been set down for two days.

What will Pell's lawyers argue?

Pell is arguing his convictions should be overturned on three grounds, including that the verdicts are "unreasonable".

At trial, evidence of the sexual abuse came from one of the former choirboys alone. Pell's other victim died in 2014 and never made a complaint to his family or police.

Pell argues the jury could not have been satisfied he was guilty beyond reasonable doubt on the complainant's evidence alone. His application for leave to appeal argues that 20 prosecution witnesses gave "unchallenged exculpatory evidence" — that is, evidence favourable to Pell.

Victorian Bar Council president Matt Collins QC says Pell's lawyers will have to convince the judges that any properly-prepared jury must have had reasonable doubt about Pell's guilt.

"It's quite a high threshold, it's not a question of the Court of Appeal just thinking it might have reached a different decision, but the Court of Appeal must be satisfied that the jury must have entertained a reasonable doubt," he said.

Pell's lawyers will also argue that the judge who presided over Pell's trial should have allowed them to play a video animation to the jury. According to Pell's legal team, this animation illustrated their argument that it was impossible Pell was alone in the priest's sacristy when he was alleged to have abused the boys.

George Pell did not wear his clerical collar when he was sentenced. ( ABC News: Fay Plamka )

The animation consisted of a number of dots moving around a floor plan of St Patrick's Cathedral. The dots depicted the movements of the many witnesses after Sunday mass.

At trial, County Court Chief Judge Peter Kidd did not allow the animation to be played to the jury.

He ruled it was based on a degree of speculation and guesswork. He also said it constituted new evidence, which could not be introduced during a closing address.

Pell's third ground of appeal is that he was not arraigned — that is, asked to enter a plea of guilty or not guilty — in the presence of all potential jurors, which is required by the law.

What needs to happen for Pell's conviction to be overturned?

Pell will first need to request leave to appeal — that is, permission for his appeal to be considered.

But his application for leave to appeal and the appeal itself will be argued at the same time, which is the Court of Appeal's usual practice to avoid separate hearings and save time.

For Pell's conviction to be quashed, his barrister will also need to convince at least two of the three Court of Appeal judges that the guilty verdicts are "unreasonable" because the jury could not have been satisfied that Pell was guilty beyond reasonable doubt.

The judges will need to make their own assessment of the evidence given at trial and then decide whether it was open to the jury to be satisfied Pell was guilty beyond reasonable doubt.

"So whereas a jury has to reach a unanimous verdict in most circumstances, in the Court of Appeal it's a majority decision, so if two judges are of one mind and another judge dissents, the two judges' decision stands," Dr Collins said.

That means they will rule on whether the jury should have had a reasonable doubt Pell was innocent, not that they could have.

Dr Collins says he expects the judges will have already read the transcripts of what happened in the trial, and reviewed all the exhibits that were put before the jury.

Justice Anne Ferguson, Justice Chris Maxwell and Justice Mark Weinberg are hearing the appeal. ( Supplied: Supreme Court of Victoria )

The judges must bear in mind that the jury had the benefit of seeing and hearing the witnesses as they gave evidence.

If they do rule that the guilty verdicts are unreasonable, Pell will be acquitted and he will be immediately released from jail.

But if the judges find that the guilty verdicts were reasonable and instead accept the second or third grounds of his appeal, a new trial could be ordered.

Will Pell be brought to court for the appeal hearing?

Pell has requested to be present at the hearing.

But that could mean he will appear via video link rather than in person.

If Pell does attend personally, he will be transported to court in a prison van and held in the court's cells before and after the hearing.

He won't be represented by the same lawyer as he was during the trial — the high-profile defence barrister Robert Richter QC.

Mr Richter stepped down from Pell's legal team in March, explaining he was "too angry and upset at the outcome to bring the objectivity that an appeal requires".

"I will not be arguing the appeal myself, simply because I believe the Cardinal deserves someone who can be dispassionate enough to present the case to the Court of Appeal," he said.

Leading Sydney silk Bret Walker SC will instead lead the case to have Pell's conviction overturned.

When will the Court of Appeal announce its decision?

It's anyone's guess.

Dozens gathered outside court to watch the verdict live when it was handed down in December. ( AAP: Daniel Pockett )

The Court of Appeal can deliver its decision on the day of a hearing or reserve its judgement to a later date.

Dr Collins says he expects the judges will retire to consider their verdict.

"It's a very efficient court, the Court of Appeal. I would expect it would be measured in months, or weeks to months, rather than any longer period."

Editor's note: On Tuesday April 7, 2020, the High Court in a unanimous decision upheld Cardinal Pell's appeal and quashed his convictions on all five charges.