The Vatican's former number-three, Cardinal George Pell, will on Wednesday begin his appeal against convictions for "brazen" sex attacks on two choirboys in the 1990s. 

Pell, who managed church finances and helped elect two popes, argues the verdict was "unreasonable," that jurors were not shown vital evidence, and that he was not asked to enter a plea in front of the panel.

The most senior figure in the Catholic church to be convicted of historical child sexual offences, Pell was sentenced in March to six years in prison.

He was found guilty on five counts, including oral rape and molesting the two 13-year-olds in the sacristy of St Paul's Cathedral when he was the Archbishop of Melbourne.

One of his victims died of a drug overdose in 2014 and never disclosed the abuse.

Trial Judge Peter Kidd described Pell's crimes as "brazen," "nasty" and a "breathtakingly arrogant" abuse of power.

A second set of charges against the 77-year-old - stemming from alleged abuse in a swimming pool in Ballarat, Victoria, in the 1970s - was dropped earlier this year.

Pell's appeal at Melbourne's Supreme Court  - which will be live streamed to the public - has been greatly anticipated by his supporters and high-profile commentators who say he was made a scapegoat for the broader sins of the Catholic church and did not receive a fair trial. 

The case will be presided over by three judges - Chief Justice of the Supreme Court of Victoria Justice Anne Ferguson, President of the Court of Appeal Justice Chris Maxwell and Justice Mark Weinberg.

The judges could determine Pell's fate this week or could defer their decision until a later date. The verdict does not have to be unanimous but must have the support of two of the justices. 

Pell is not required to appear at the appeal hearings on June 5 and 6, and it is unclear if he will show up or possibly appear via video link from jail.