The string of historical child sexual abuse charges laid against Cardinal George Pell has plunged the case into unknown legal territory. 

In the case of Cardinal Pell, leading Australian extradition expert Professor Donald Rothwell has previously outlined three possible scenarios. 

He said Cardinal Pell could refuse to step outside the Vatican walls to avoid prosecution, as there is no extradition treaty with Australia.

However, Professor Rothwell believed that was highly unlikely. 

He said Cardinal Pell had so far co-operated with police, sought to engage in legal proceedings and vehemently denied any allegations of misconduct.

If the Cardinal did not agree to return, Professor Rothwell said it would be difficult for Victoria Police to force him.

Australia could appeal to Italy to make the extradition request on its behalf.

Under this scenario, clergy wanted on charges in Australia could be arrested by Italian authorities if they left the holy city.

"The mere fact there is no extradition treaty does not create an impossible barrier to extradition because Australia has diplomatic relations with the [Vatican]," Professor Rothwell told Fairfax Media last month.

"There are legal relations between the two countries [Australia and Italy], so theoretically, an extradition request could be made."

Under another scenario, Pope Francis could order Cardinal Pell to return to Australia.

Under the church's international code of canon law, the Pope is the supreme legislator and is in charge of the church legally.

He has also previously backed Cardinal Pell and endorsed him for his high-ranking role of Secretariat for the Economy, commonly described as the Vatican's financial boss.

Traditionally, the decision to lay charges is based on whether it is "more likely than not"  they would result in a successful prosecution. 

In historic sex crime investigations, police must consider the severity of the allegations, the credibility of witnesses, potentially corroborating evidence, and a lack of forensic evidence. 

Cardinal Pell, Australia's highest ranking Catholic official, was interviewed by three members of Victoria Police in Rome last October. 

The 75-year-old took part voluntarily.

Cardinal Pell has always vehemently denied sex abuse allegations made against him.

There has been only one other known case in history where a senior Vatican official was charged with sexually abusing children. 

Polish former archbishop Jozef Wesolowski would have been the first high-ranking church official to go on trial for paedophile charges but he died awaiting trial in August 2015.

The 67-year-old was found dead in his Vatican City residence after he was placed under house arrest in September for 2014 following allegations he sexually abused boys while serving in the Dominican Republic. 

The Wesolowski case sparked global controversy after it was widely reported the Vatican had learnt of the allegations and helped him to leave the country before he could be investigated.

The Vatican then invoked diplomatic immunity, protecting Wesolowski from facing trial in the Dominican Republic.

Wesolowski lost his diplomatic immunity after he was defrocked in late 2014.

The Vatican decided to try him at home but subsequently said he could face charges elsewhere after the Vatican's case against him concluded.