George Pell could walk from court today or be sent back to prison to serve the rest of his six-year term, depending on the outcome of his appeal against his child sex abuse convictions.

The Victorian Court of Appeal will rule on whether to overturn the former adviser to the Pope's convictions for sexually abusing two choirboys when he was archbishop of Melbourne in the 1990s.

A jury found Pell guilty last December of sexual penetration of a child under 16 and four counts of committing an indecent act with a child.

His appeal was heard over two days in June and after scrutinising the arguments the three judges have now made their decision.

Chief Justice of the Supreme Court Anne Ferguson, President of the Court of Appeal Justice Chris Maxwell and Justice Mark Weinberg will hand their decision down at 9:30am today.

The judgment will be heard in one of the Supreme Court's biggest rooms and an extra room has been set aside for journalists who cannot fit in the court to watch the proceedings live.

It will be live streamed on the Supreme Court of Victoria website and broadcast here on the ABC News website, on the ABC News Channel and on ABC NewsRadio on television.

Here's what could happen.

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

What would need to happen for Pell to walk free?

Pell's legal team argued the appeal on three grounds, but the argument that the verdicts were unreasonable was their main focus.

His lawyers argued there was insufficient evidence for a jury to be satisfied beyond reasonable doubt that he was guilty of sexual abuse.

Pell's barrister, Bret Walker SC, told the appeal hearing that the jury could not have excluded a reasonable doubt because of the many improbabilities contained in the victim's evidence.

The defence team listed 13 reasons why the offending could not have occurred, including issues with Pell's location in the cathedral at the time of the offending and with the dates of the offences.

The prosecution lawyer, Christopher Boyce QC, argued Pell's victim was a "witness of truth" and Pell's sex abuse convictions were "unimpeachable" and should be upheld.

Pell was convicted of abusing two choirboys inside Melbourne's St Patrick's Cathedral in the 1990s. ( ABC News: Danielle Bonica )

It's a high bar for the judges to acquit Pell.

They would need to be convinced that any properly prepared jury ought to have entertained a reasonable doubt, not that they merely could have.

But if they are convinced and the convictions are quashed, Pell could walk free.

"If the Court of Appeal substitutes a verdict of acquittal, I think the overwhelming likelihood is that Cardinal Pell will be released immediately," says the president of the Victorian Bar, Matthew Collins QC.

What would cause a retrial?

A retrial would be ordered if the three appeal judges agree that there were errors of law made in the running of Pell's trial by County Court Chief Judge Peter Kidd.

Pell was arraigned via video link, but his lawyers argued he should have been allowed to plead not guilty in person before the jury.

They also argued the judge erred in refusing to allow a video animation of St Patrick's Cathedral where the abuse occurred to be played to the jury during the defence's closing address.

The animation consisted of a number of dots moving across the floor plan of the cathedral, representing the alleged movements of witnesses after Sunday mass.

But Justice Kidd ruled the video constituted new evidence and did not allow it to be played.

These two grounds for appeal were described by Pell's barrister as "fall-back" options if the judges disagreed the verdicts were unreasonable.

But if the judges agree one or both of the errors meant the cardinal was deprived of a fair trial, the likely outcome is more time in court.

"If the Court of Appeal finds that the errors were material then the likelihood is that the verdict would will be set aside and a retrial ordered," Dr Collins says.

But a retrial isn't guaranteed. The judges will also consider the expense and burden of a retrial and the length of time Pell has already served in prison compared with a likely sentence if found guilty again.

Pell was found guilty at the end of a second trial after the first jury to hear his case couldn't reach a verdict. ( ABC News: Fay Plamka )

And Pell's case is incredibly high-profile, so they would also weigh up any risks to the cardinal receiving a fair trial.

"It might be argued that it would be difficult to arraign a jury that did not already have a view as to Cardinal Pell's guilt or innocence," Dr Collins says.

If the court does order a new trial, it will be up to the Director of Public Prosecutions to decide whether it should go ahead.

What if the appeal is dismissed?

If the Court of Appeal judges aren't convinced Pell's convictions should be overturned, the jury's guilty verdict would be affirmed.

Pell would be transported back to prison to continue serving his six-year jail term.

He has indicated he does not want to appeal against the length of his sentence.

The 78-year-old spent his birthday behind bars in June and would remain there until at least the age of 81, before becoming eligible for parole.

Will this be the end of it?

Almost certainly not.

If the court finds the jury got it wrong and Pell's convictions are quashed, Victorian prosecutors could appeal against that decision in the High Court.

The same goes for Pell's legal team if the appeal is dismissed entirely — they could take that decision to the High Court.

Either way, the country's top court would first have to decide there are grounds for the case to be heard.

Dr Collins says the chances of the case being escalated are "reasonably high".

"Whichever party fails on the appeal I expect would be looking very anxiously at whether there were errors which could potentially be corrected by the High Court," he says.

"Obviously in a case of this significance, with the sorts of resources that both sides have applied to the matter, I think it's all but certain that they'll be looking at those options."

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.