Jailed Australian Cardinal George Pell launched an appeal Wednesday against his landmark conviction for child sex abuse, branding the charges against him as "bizarre" and "impossible."

Pell, 77, left his prison cell under police guard to attend the two-day hearing, battling his conviction in December on five counts of sexually 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 - was sentenced in March to six years in prison, becoming the most senior Catholic ever jailed for child sex abuse.

Wearing his clerical collar and a black suit, Pell took notes of the proceedings as his lawyer, Bret Walker, detailed a long "catalogue" of elements he said should have prevented his client's conviction.

Walker argued mainly that Pell's jury verdict was unreasonably based on the testimony of a single surviving victim and that the judge in his trial unfairly disallowed defense evidence.

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

Walker said the timing of the assaults in the cathedral and its sacristy following Sunday masses was "impossible" given the dates and Pell's publicly verified movements on the days in question.

Walker said Pell was mingling with congregants at the western door of the cathedral when the abuse purportedly took place. "The word is alibi," Walker said, adding that the entire case against the cardinal amounted to a "bizarre unlikelihood."