Australia's highest court on Tuesday unanimously dismissed the convictions of the most senior Catholic found guilty of child sex abuse. Cardinal George Pell soon will be released from Barwon Prison outside Melbourne after serving 13 months of a six-year sentence.

Pope Francis' former finance minister was convicted by a Victoria state jury in 2018 of sexually abusing two 13-year-old choirboys in a back room of St. Patrick's Cathedral in Melbourne in December 1996 while he was archbishop of Australia's second-largest city.

Pell was also convicted of indecently assaulting one of the boys by painfully squeezing his genitals after a Mass in early 1997.

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Pell was regarded as the Vatican's third-highest ranking official when he voluntarily returned to Melbourne in July 2017 determined to clear his name of dozens of decades-old child abuse allegations.

Cardinal George Pell seen March 5, 2018, in Melbourne, Australia. Getty

All the charges were dropped by prosecutors or dismissed by courts in preliminary hearings over the years except the cathedral allegations.

Pell was tried on the charges twice in 2018, the first ending in a jury deadlock. He did not testify at either trial or at the subsequent appeals.

But the juries saw his emphatic denials in a police interview that was video recorded in a Rome airport hotel conference room in October 2016.

"The allegations involve vile and disgusting conduct contrary to everything I hold dear and contrary to the explicit teachings of the church which I have spent my life representing," Pell read from a prepared statement.

He also pointed out that he had established a world-first compensation scheme for victims of clergy, the Melbourne Response.

As police detailed the abuse allegations, Pell responded: "Absolutely disgraceful rubbish. It's completely false. Madness."

Pell was largely convicted on the testimony of one of the choirboys, now in his 30s with a young family.

He first went to police in 2015 after the second victim died of a heroin overdose at the age of 31. Neither can be identified under state law.

Much of the two-day hearing at the High Court last month had focused on whether the jury should have had a reasonable doubt about Pell's guilt and whether he could have time to molest the boys in five or six minutes immediately after a Mass.

The Victorian Court of Appeal found in a 2-1 majority in August that Pell had had enough time to abuse the boys and that the unanimous guilty verdicts were sound. But in the decision announced by High Court Chief Justice Susan Kiefel, the seven judges found that the appeals court was incorrect.

The two appeals court judges who had upheld the convictions had found the former choirboy a "compelling witness," the High Court said in a statement.

But the two judges' "analysis failed to engage with the question of whether there remained a reasonable possibility that the offending had not taken place, such that there ought to have been a reasonable doubt as to the applicant's guilt," the statement said.

Director of Public Prosecutions Kerri Judd told the High Court last month that the surviving choirboy's detailed knowledge of the layout of the priests' sacristy supported his accusation that the boys were molested there.

Pell's lawyers argued that Pell would have been standing on the cathedral steps chatting with churchgoers after Mass when his crimes were alleged to have occurred, was always with other clerics when dressed in his archbishop's robes, could not have performed the sexual acts alleged while wearing the cumbersome garments and could not have abused the boys in the busy priests' sacristy without being detected.

The High Court referred to the "unchallenged evidence" of witnesses in the trial to Pell's practice of talking to the congregation on the cathedral stairs, church practice that required him to be accompanied in the cathedral while robed and the "continuous traffic in and out of the priests' sacristy" as causes for reasonable doubt.

Pell's lawyer Bret Walker told the High Court that all that the prosecution had to do at his trial and appeals court hearing was to prove that Pell being left alone while robed or not talking with congregants after Mass was "possible" to prove guilt beyond reasonable doubt.

"That ... is a grotesque version of the reversal of onus of proof, if all the Crown has to do is to prove the possibility of something," Walker said.

Judd argued that the charges were proved beyond reasonable doubt.

The High Court statement said: "The High Court found that the jury, acting rationally on the whole of the evidence, ought to have entertained a doubt as to the applicant's (Pell's) guilt with respect to each of the offences for which he was convicted."