As the first day of George Pell's appeal hearing closes in Canberra, all eyes — in Australia and overseas — are on the High Court as it decides the future of the former advisor to the Pope and most senior Catholic to be convicted of child sex offences.

78-year-old Pell is serving a maximum six years in jail after a jury found him guilty of sexual offences against two choirboys in the late 1990s when he was Archbishop of Melbourne.

On Wednesday, the High Court took an interest in what is essentially a technicality: should the court of appeal have watched the videotaped evidence given by the victim at trial.

Justice Virginia Bell questioned why the full bench had not been invited to watch the same evidence.

"Why are we being taken to selected portions of the transcript?" Justice Bell asked.

"And why aren't we getting the whole transcript?"

Pell's lawyers make 'reasonable doubt' argument

Pell's lawyers are hoping to convince the High Court the jury made a mistake and his conviction should be quashed.

They argued on Wednesday that the prosecution did not reach the bar of proving "reasonable doubt", particularly when it came to the five to six minute window in which the abuse is said to have occurred.

Beginning his case, Bret Walker SC took to his feet with the long-running argument that Pell could never have committed the abuse in the midst of a "hive of activity" after a Solemn Mass at St Patrick's Cathedral.

"These are not deserted premises, these are not areas where nothing is going on," Mr Walker argued.

He said there was a "sheer unlikelihood" that events would have aligned to ensure a five minute hiatus of activity for Pell to have been alone with the choirboys.

Mr Walker also argued the judges who believed the victim's evidence as truthful did not properly consider testimony by other witnesses that suggested it was "unlikely" if not "improbable" that Pell was alone with him and his friend.

Taking issue with the Court of Appeal's reasoning for upholding the guilty verdicts, Mr Walker argued reasonable doubt can co-exist with belief in the victim's testimony.

"Belief in a complainant does not eliminate the possibility of co-existent reasonable doubt as to guilt," Mr Walker submitted.

"The people we believe may be wrong — either lying ... or simply wrong".

But lawyers for the prosecution have told the High Court that Pell's case is "weak" and glosses over evidence that supports the victim known as 'A'.

Outside court on Wednesday morning, the lawyer acting for the father of one of the victims, who has since died, said her client was feeling apprehensive about the day's challenge.

"He's understandably quite anxious and unsettled by this," Lisa Flynn said.

"However, he also remains very hopeful and also confident that the High Court will agree with the Court of Appeal majority and that the convictions against George Pell will be upheld.

"The High Court won't be looking at every piece of evidence in the case, that was the jury's job over a number of weeks.

"The High Court will really be looking at the evidence as a whole, and looking at specific legal points in terms of the appeal over the next two days."

How did the case get to the High Court?

Pell has been sentenced to six years in jail for the sexual abuse of two choirboys in the 1990s. ( ABC News: Fay Plamka )

Pell's case is extraordinary because of who is involved and what is at stake. But, on another level, it is a standard historical sexual abuse case based on the word of a victim against that of a person of authority, years after the events.

The case revolves around sexual offences against two choirboys who snuck away from the post-mass procession and were caught by Pell swigging altar wine in the priest's sacristy.

This was then followed by a second offence in a momentary encounter with Pell's accuser weeks later.

A complication in the Pell case was that the second boy had died before the matter was reported to police 2015 and, when asked in 2001 by his mother, he had denied ever suffering abuse.

Nonetheless, a jury convicted Pell of five charges in 2018.

Pell's lawyer Robert Richter said he was so "angry and upset" by the result he could not go on, leaving the subsequent appeals to barristers Mr Walker and Ruth Shann.

The first, in the Victorian Court of Appeal, found in a two-to-one decision that 'A' was a witness of truth and it was open to the jury to find Pell guilty beyond reasonable doubt.

It is this decision and the manner in which it was made that was the focus of the High Court challenge on Tuesday.

It is also familiar territory for the court.

High Court to examine the use of the 'M Test'

The High Court will consider the decisions made by Chief Justice Anne Ferguson, Justice Chris Maxwell and Justice Mark Weinberg. ( Supplied: Supreme Court of Victoria )

At the heart of the submissions put to the court so far is something called the "M Test", from the 1994 High Court case M v the Queen.

It requires appeal courts to make an independent assessment of the evidence, and then to decide whether it was open to the jury to arrive at its verdict beyond reasonable doubt. It is meant to protect jury decisions, except in situations where there has been a mistake.

Just how the Victorian Court of Appeal carried out that task is what is at issue in the High Court.

The case, put forward by Pell's lawyers in the trial, the appeal and the now the High Court, relies heavily on so-called "solid obstacles" to conviction.

Witnesses involved in church activities at the time gave evidence that raised a series of questions about whether the crime was possible, and whether Pell would have had an opportunity to commit the offences.

The evidence ranged from the centuries-old tradition of never leaving an Archbishop alone while robed; to whether Pell could have even committed the offences while robed; and whether the scene of the crime, the Priest's Sacristy, would have been vacant for the five or six minutes the offending was said to have taken place.

George Pell is the most senior member of the Catholic Church to be found guilty of child sexual abuse. ( Reuters: Max Rossi )

A great deal was also made of how Pell would spend time on the church steps after mass, talking to worshipers.

His lawyers said that all of this evidence about the practices and routines at St Patrick's Cathedral in Melbourne effectively provided Pell with an alibi, and showed a lack of opportunity.

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But two of the judges on appeal, Chief Justices Anne Ferguson and president Chris Maxwell, disagreed, saying they were not convinced.

"No witness could say with certainty that these routines and practices were never departed from," they said in their decision.

Pell's lawyers now argue that the Court of Appeal did not apply the M Test correctly, by treating each issue in isolation.

"The point is that the majority did not independently weigh the combined effect of the evidence," Pell's lawyers said in their submission.

They preferred the dissenting opinion of Justice Mark Weinberg, who found there were compounding improbabilities established by the evidence.

Onus of proof reversed, Pell's lawyers argue

A major thrust in the case put forward by Pell's lawyers is that the appeal ruling reversed the onus of proof and required Pell to establish actual innocence.

They have also accused the appeal judges of basing their decision on belief in 'A' alone.

Not surprisingly, in its written submissions to the High Court the prosecution has refuted that.

"The majority did not reason on the basis of 'belief' in A," they said.

"Rather they 'reviewed the whole of the evidence' and made an assessment of A's credibility and reliability."

The prosecution said the account by 'A' was supported by evidence, including his account of the offence happening in the priest's sacristy.

The sacristy was being renovated at the time — a detail not many other witnesses could recall.

A decision on Pell's appeal is likely to be months away. ( AAP: Paul Miller )

One of the curious things in the case was that the Victorian Court of Appeal did not stick to the transcripts, as would normally happen.

The three judges watched videos of the victim and several other witnesses giving their evidence, in the same way the jury would have. They also visited the cathedral.

The court asked for extra submissions on whether the M Test allows such a thing, or restricts appeal courts to the transcripts.

On Wednesday, the High Court heard the arguments from both sides.

But a decision is likely to be months away.

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.