After less than four months in home detention, former Catholic archbishop Philip Wilson has had his conviction for concealing child sex abuse overturned.

Key points: Philip Wilson had his conviction for concealing the abuse of an alter boy overturned

Philip Wilson had his conviction for concealing the abuse of an alter boy overturned The judge said the rule of law comes before public and media opinion

The judge said the rule of law comes before public and media opinion One survivor said this is the "end of the road" for concealment convictions

He was the highest-ranking Catholic in the world convicted of the crime, in what many people described as a landmark decision that could have opened the court doors to a flood of other allegations of church concealment.

So how has that conviction now been quashed?

In short, because it is a difficult charge to prove.

The charge

It was alleged that in 1976 a 15-year-old altar boy, Peter Creigh, went to Father Philip Wilson on two occasions and told him that four years earlier he had been abused by a fellow priest, James Fletcher.

Mr Creigh said he did not hear anything about the matter again and he held on to the allegations until coming forward to his family in 2009.

He told the Bishop of the Maitland Newcastle Diocese Michael Mallone about the abuse in 2010 before taking the matter to police in 2013.

In that time the offending priest, Fletcher, had been charged in 2004 with other child sex abuse crimes and was convicted before dying in jail in 2006.

But on hearing Mr Creigh's evidence, the Director of Public Prosecutions decided Father Wilson also had a case to answer.

As a result, the former Adelaide archbishop was charged with failing to bring Mr Creigh's allegation forward to police, not in 1976, but when Fletcher was charged in 2004.

To get the conviction, the Crown had to prove that Mr Creigh actually told Father Wilson about the abuse in 1976, and that Father Wilson believed the allegation.

But could this be proved beyond reasonable doubt?

Memory issues

Father Wilson was a "very honest and forthright" witness, Judge Ellis said yesterday. ( AAP: Alan Porritt )

In May, Local Court Magistrate Robert Stone convicted Father Wilson after accepting that Mr Creigh's memory of events was accurate.

But on Thursday, District Court Judge Roy Ellis was far more critical.

He acknowledged Mr Creigh gave an honest account but he questioned the accuracy of his memory given the length of time that had elapsed.

He also pointed to the evidence of memory expert Professor Richard Kemp who raised the potential for false memories.

"It is not inconceivable that in looking back Mr Creigh convinced himself that he had complained, rather than asking himself why he didn't complain," Judge Ellis told the court on Thursday.

Judge Ellis also took issue with 12 inconsistences Mr Creigh made during his statements to authorities, including whether the conversation with Father Wilson was before or after Easter, whether it was between 15-20 minutes or 30-40 minutes or whether he referred to Father Wilson as "Philip" or "Father Wilson".

"The inconsistencies neither individually nor in combination prove that the alleged conversation did not take place, but they are capable of raising reasonable doubt," Judge Ellis told the court.

38 years later

Since being charged in 2015, Father Wilson has never ruled out the conversations with Mr Creigh took place, but has maintained he has no memory of them.

So the question became would Father Wilson remember the allegations made to him in 1976 when Fletcher was charged in 2004.

The Crown asserted he "must have" because it is unlikely he would ever forget such a conversation given the serious nature of the details.

Peter Creigh is surrounded by supporters on the day of Philip Wilson's conviction. ( ABC News: Brooke Wylie )

But Judge Ellis was not so sure.

"The appellant Philip Wilson was arrested and charged with the current offence on 17th of March 2015," he said.

"That arrest meant that essentially after 38 or 39 years the appellant had to ask himself whether he recalled a conversation with Mr Peter Creigh in 1976, and if he did, whether he had remembered it in 2004."

He found it could not be proven beyond reasonable doubt that Father Wilson remembered Mr Creigh's allegations.

Did he believe it?

The Crown also had to prove Father Wilson believed the allegations put to him by Mr Creigh.

It argued he "must have", claiming Father Wilson knew of three other separate allegations about Fletcher from other boys, including one who allegedly confided in him during confession.

During his trial, Father Wilson maintained that even if true, this would not be enough to convince him of a person's guilt.

He said he would only believe an allegation if he heard a confession or if it was proven in a court of law.

When convicting Father Wilson in May, Magistrate Stone described his answers on this point as "evasive", likening him to a "cat on a hot tin roof" under questioning.

But on Thursday Judge Ellis described Father Wilson as a "very honest and forthright" witness.

He said he appreciated that Father Wilson would keep an "open mind", saying it "is the appropriate response from any intelligent, reasonable and thoughtful individual".

"While it is undoubtably correct to conclude that a person hearing the same type of allegation against the same person by three or four individuals would be more likely to believe the allegation, it does not necessarily follow that would in fact be the case or that … Philip Wilson did in fact form a belief as to the guilt of the alleged perpetrator," Judge Ellis said.

"There is no direct evidence, or even evidence of conduct of the appellant, that assists in determining what belief was actually formed, if in fact the appellant did form a belief in relation to Mr Creigh's allegation."

Dead priest's evidence deemed significant

During Thursday's appeal verdict, Judge Ellis also gave the story of Hunter Valley priest Glen Walsh more weight.

The court heard Mr Walsh became aware of an allegation of child abuse against Fletcher in 2004, but he was directed to hide the allegation by Bishop Malone.

In a statement to police Mr Walsh said he sought advice from Father Wilson, who said he should encourage the victim to speak to police, go to authorities himself and provide appropriate pastoral care.

Judge Ellis said this information was significant in determining how Father Wilson would respond to an allegation of sexual abuse.

"It seems inconceivable that Philip Wilson would give this correct and appropriate advice to Father Walsh … when he Philip Wilson was knowingly refusing to follow the same advice in relation to another boy who made a complaint against James Fletcher to him."

Mr Walsh was never able to give evidence in Father Wilson's trial, having taken his own life just weeks before it began.

Where to from here?

In his opening address in court on Thursday Judge Ellis said Father Wilson should not be convicted on the "direction of public opinion" or on the sins of the Catholic Church as an institution, but rather the rule of law.

"In practice, complying with the legal principles may well result in a verdict that is perhaps inconsistent with media or community expectations, given more recent trends in public and media opinion," he said.

"But if the verdict is a true representation of justice … then it is community or media expectations that must be dashed, not the hopes of an individual that he or she will receive a fair trial."

Australian priest, lawyer and academic Father Frank Brennan has always argued that Father Wilson was the "wrong test case" when it comes to allegations of concealing abuse.

He has called for the section of NSW law under which Father Wilson was charged, Section 316, to be scrapped and comprehensively overhauled, describing it as an "untested, vague, problematic provision".

Abuse survivor Peter Gogarty speaks outside court after Philip Wilson's sentencing. ( ABC News: Nancy Notzon )

Meanwhile Thursday's decision has sent shockwaves through the networks of child sex abuse survivors who have been left wondering if any individual will ever be convicted of a concealing offence.

The royal commission into child sex abuse identified the widespread and systematic cover-up of abuse, yet no individual has ever been held to account.

"It seems now almost impossible to ever be able to get a conviction on a concealed offence unless a person admits to it, and even then they can say they were told but didn't believe it," survivor Peter Gogarty said.

"If you go by Judge Ellis's reasoning, Philip Wilson would have had no obligation to come forward with the allegation until there was an admission or it was proven in a court of law.

"Will anyone ever be convicted? I don't think we can do it now. We're at the end of the road."

The Director of Public Prosecutions is now believed to be considering whether to challenge Judge Ellis's decision in the NSW Supreme Court.