MELBOURNE — Jailed Australian Cardinal George Pell arrived in court under heavy police guard on Wednesday, beginning an appeal against his conviction for child sex abuse.

Pell, 77, was found guilty in December on five counts of assaulting two choirboys in Melbourne's St Patrick's Cathedral in 1996 and 1997.

The Vatican's former number three -- who managed the church's vast finances and helped elect two popes -- was sentenced in March to six years in prison.

During proceedings he sat -- flanked by police officers -- with his hands interlaced or taking notes with pen and paper.

His lawyers say his conviction was unreasonably based on the testimony of a single surviving victim and that the judge unfairly disallowed defence evidence.

Pell's second victim died of a drug overdose in 2014 and never disclosed the abuse.

In court documents, Pell's lawyers claimed "a catalogue of at least 13 solid obstacles in the path of a conviction".

Pell wore his clerical collar and a black coat for the appeal, which is being heard by three judges of Victoria State's Supreme Court: Chief Justice Anne Ferguson, President of the Court of Appeal Chris Maxwell and Justice Mark Weinberg.

The cardinal has always maintained his innocence and supporters have claimed he was being punished unfairly for the Catholic church's multiple cover-ups of paedophile priests.

The hearing is scheduled for Wednesday and Thursday before the three judges make a decision on his case, a process that could take several weeks.

They could reject the appeal, order a retrial or acquit Pell. Any ruling could be appealed further to Australia's High Court.

Prior to Wednesday's hearing, the three judges visited Saint Patrick's cathedral to understand the evidence that was considered by the jury, a court spokesman said.

The defence argues that the timing of the assaults were "impossible" given the dates and his publicly verified movements within the cathedral.

Pell's lawyer Bret Walker opened his appeal by dissecting the "impossibility" of the abuse, describing elements of the incidents as "bizarre unlikelihood".

He claimed that Pell was mingling with congregants at the western door of the cathedral when the abuse purportedly took place. "The word is alibi," Walker said.

The prosecution said "the evidence from a handful of witnesses suggested that certain scenarios, such as the Archbishop being alone and robed, were unlikely", but not impossible.

The appellants also claim it was not possible for the two choirboys to have left their group unnoticed or for the sexual assault in the sacristy to have gone undetected when the cathedral was busy following mass.

The victim's testimony and cross-examination were given to the jurors and judge alone. 

A video of that cross-examination is expected to be reviewed by the appeal judges.

A first trial in the case last year ended in a hung jury. He was convicted in December at the end of a second trial.

Both trials were hidden from the public until a wide-ranging gag order was lifted in February after a second tranche of charges against Pell, involving alleged incidents in a swimming pool in his home town of Ballarat in the 1970s were dropped.

Pell, an outspoken conservative, enjoyed a meteoric rise through the ranks of the church, becoming Australia's most senior Catholic.

Former prime minister John Howard was one of several people to pen a glowing character reference for Pell ahead of his sentencing. — AFP