Convicted paedophile George Pell had his appeal against child sex abuse charges rejected by an Australian court.

Once the Vatican's third-ranking official, the 78-year-old was sentenced this year to six years in jail for sexually assaulting two 13-year-old choirboys at a Melbourne cathedral in the 1990s.

"He will continue to serve his sentence of six years imprisonment," said Chief Justice Anne Ferguson, dismissing a series of appeals from the cardinal's lawyers.

Pell is the most senior Catholic convicted of child sex abuse.

A large crowd of victims, advocates, lawyers and media gathered outside the court ahead of the verdict, with a long queue to enter the building forming along the street.

After more than two months of deliberations, a three-judge appeals panel at the Supreme Court of Victoria handed down its decision.

Pell's lawyers raised 13 objections to his conviction, casting doubt on everything from the physical possibility of Pell removing his robes to the credibility of the main witness.

They argued the verdict was "unreasonable".

The judges unanimously dismissed two grounds of appeal, which argued that there were errors in the way the trial was run.

Two of the three judges turned down Pell's primary ground of appeal, that the jury's verdict was unreasonable.

The case was unusual in that it relied heavily on the closed-door testimony of the sole surviving victim, who cannot be named for legal reasons.

The verdict could have wide-ranging implications for sexual assault cases that rely on the account of a single victim.

On Monday, the father of the second victim - who died of a drug overdose in 2014 - expressed hopes that "justice would prevail" and that the ordeal would soon be over.

"He just wants closure so he can try to get on with his life and stop thinking about it every single day," his lawyer Lisa Flynn said.

A spokesman for Pell has said his legal team is studying the judgment to determine whether to appeal to the High Court.

In a statement, the Vatican acknowledged the court's decision to uphold Pell's conviction but said he has a right to another appeal.

"As the proceedings continue to develop, the Holy See recalls that the Cardinal has always maintained his innocence throughout the judicial process and that it is his right to appeal to the High Court," the statement said.

"At this time, together with the Church in Australia, the Holy See confirms its closeness to the victims of sexual abuse and its commitment to pursue, through the competent ecclesiastical authorities, those members of the clergy who commit such abuse," it said