An Australian archbishop has become the most senior Roman Catholic cleric in the world to be convicted of covering up child sex abuse.

Archbishop of Adelaide Philip Wilson faces a potential two years in prison when he is sentenced next month.

Magistrate Robert Stone handed down the verdict in Newcastle Local Court, north of Sydney, following a magistrate-only trial.

Wilson, 67, had pleaded not guilty to concealing a serious crime committed by another person – the sexual abuse of children by paedophile priest James Fletcher in the 1970s.

Mr Stone told the court that Wilson had concealed the abuse of two altar boys in the Hunter Valley region, north of Sydney, by Fletcher by failing to report the allegations to police.

The judge said he was satisfied that one of the altar boys, Peter Creigh had been a “truthful and reliable” witness.

Wilson was released on bail until he appears at a sentencing hearing on June 9. Prosecutors will argue for a custodial sentence.

In a statement issued by the Catholic Church, Wilson said he was disappointed by the conviction.

“I will now have to consider the reasons and consult closely with my lawyers to determine the next steps,” he said.

Prosecutor Gareth Harrison had submitted that Wilson was involved in a cover-up to protect the church’s reputation and there were doubts about his honesty.

Harrison argued that, in Wilson’s mind, victims came second.

Wilson, who is suffering from the early stages of Alzheimer’s disease but maintains that medication has helped his memory, told the court during his trial last month that he could not remember Mr Creigh and another altar boy telling him in 1976 that they were abused by Fletcher.

The court has ordered that Mr Creigh can be named in media reports on his evidence, but the second accuser cannot be named for legal reasons.

The prosecution argued that Wilson failed to give details to police about a serious crime after Fletcher was arrested in 2004 and went on trial for preying on another boy.

Fletcher was found guilty of nine counts of child sexual abuse and died in prison of a stroke in 2006 while serving an almost eight-year sentence.

Defence lawyers had argued that Wilson could not be found guilty because the case was circumstantial and there was no evidence to prove the archbishop was told about the abuse, believed it was true, or remembered being told about it.

Defence lawyer Stephen Odgers urged the magistrate to take into account that in the 1970s a priest having sex with a boy was not considered a serious indictable offence, the legal basis for the concealment charge against Wilson.

Mr Odgers told the court the abuse would have been viewed as an act of indecency, not indecent assault, if the victim could not prove he had been forced to perform sex acts.

Mr Creigh told the court he had trusted that Wilson, then an assistant priest, would take action after he told him Fletcher had repeatedly abused him in 1971 when he was 10.

Mr Creigh testified that Wilson had a “look of horror” on his face when told of the abuse, that the clergyman took no action and did not tell police.

The defence did not challenge Mr Creigh’s truthfulness, but raised concerns about the reliability of his memory of what happened in 1976.

The other former altar boy said he was about 11 in 1976 when he went to confession and told Wilson that Fletcher had abused him.

The witness told the court that Wilson refused to believe him because Fletcher “was a good bloke”.

Wilson ordered the boy to get out of the confessional box and say 10 Hail Mary prayers as an act of contrition, the court was told.

Wilson gave evidence last month that he had no memory of seeing the second altar boy at all in 1976 and he would never accuse anyone in the confessional of telling lies.

Former police detective chief inspector Peter Fox, who had previously made public allegations that the church had covered up Fletcher’s crimes, said outside court that he was “delighted” by the verdict because someone in the church was being held to account.

“It’s a major turning point. It’s been very difficult for so many of these witnesses; I can’t tell you how difficult it’s been,” he said.