Cardinal George Pell is serving a six-year sentence for sexually abusing two choirboys. File picture: Tony Gentile/Reuters

Melbourne - Australian Cardinal George Pell, who is serving a six-year sentence for child sex abuse, has been sent back to jail, after the two-day appeal hearing ended on Thursday without a decision from the judges. 

The former Vatican treasurer and one-time close adviser to Pope Francis was convicted for sexually abusing two choirboys in a Melbourne cathedral in the mid-1990s. He maintains his innocence. 

The Court of Appeal in Melbourne began on Wednesday hearing Pell's application for leave to appeal and the appeal itself, both being argued at the same time, in front of three judges.

The hearing ended on Thursday, but the judges reserved their decision until a later date, which has not been announced. 

Pell, the highest-ranking Catholic figure to be convicted of child sex abuse, was sent back to jail after the hearing. He will spend his 78th birthday in solitary confinement on Saturday. 

Cardinal George Pell arrives at the Supreme Court of Victoria in Melbourne. Picture: Julian Smith/AAP Image via Reuters

Pell's lawyers are trying to overturn the conviction arguing that the jury's verdict was "unsafe and unsatisfactory," while the prosecutors told the judges on Thursday that the jury was correct to convict Pell. 

Early on, senior prosecutor Chris Boyce made a disastrous slip-up by announcing the name of Pell's victim, which is not allowed in Victoria for sexual assault cases, forcing the Supreme Court chief judge Anne Ferguson to warn him to "be careful."

A 15-second delay in the live-streaming of the proceedings via the court website helped avoid the name of the complainant from being broadcast to the world.

Boyce was visibly distressed and paused with the court watching silently in horror. It took him a few moments to recompose himself after strong encouragement from the bench.

The prosecution argued on Thursday the complainant would not have known the layout of the cathedral's sacristy, where the abuse took place, as was evident during the trial, if he had never been there.

Boyce said it was compelling evidence and "no coincidence."

On Wednesday, Pell's lawyer, Bret Walker, said that it was not possible for Pell to have committed the assaults without being detected. He also said the jury relied solely on the testimony of one surviving victim, as opposed to exculpatory evidence from 20 others who also gave evidence at the trial. 

Boyce defended the integrity of the complainant on Thursday, saying the evidence given by the victim was so "moving" that "any doubt one might have had about the account ... is relieved."

"He was clearly not a liar, not a fantasist. He was a witness of truth," he said. 

Protester Joe Mitchell, 83, drove more than 1 000 kilometres from his home in Newcastle, New South Wales, to the Victoria state Court of Appeal in Melbourne. Picture: Rod McGuirk/AP

Justice Chris Maxwell, one of the judges presiding over the appeal, said the court would have to consider submissions that the man was a liar and a fantasist.

"If it's a fantasy at some point you'd expect the cracks to appear," said Boyce. 

Walker had argued on Wednesday that Pell's clerical robes made the abuse "physically impossible." 

But Boyce suggested on Thursday the judges try on the robes and see for themselves whether the evidence holds up. 

Apart from arguing that Pell's convictions should be overturned on grounds of unreasonableness, his lawyers also argued two other grounds of appeal, both of which will likely lead to a retrial.  

One is that Pell was not arraigned in the presence of the jury panel, and the other that Pell's defence team should have been allowed to show a video animation to the jury. 

In December, Pell was unanimously convicted by a jury on five charges: one count of sexual penetration of a child under 16, and four counts of committing an indecent act with or in presence of a child under 16. He was sentenced in March. 

An earlier jury had been discharged in September after it could not reach a verdict. 

Pell had just become archbishop of Melbourne when the sexual assaults took place in St Patrick's Cathedral. 

The Vatican has banned Pell from public ministry or having contact with children, and has launched its own investigation into his convictions.

It is not certain when the appeal court judges' verdict, which could be majority or unanimous, would come. 

It could take several weeks or even months.