Fr John Gerard Nestor, who attended Sydney's St Patrick's Seminary with Mr Abbott in the 1980s, was a priest in the Wollongong diocese in NSW when he was charged with the indecent assault of a 15-year-old altar boy in 1991.
Fr Nestor, then aged 44, was convicted in Wollongong Local Court on February 18, 1997, and sentenced to 16 months in jail, with the magistrate describing the case as a ''gross breach of trust''.
In court, the priest admitted he had - while dressed in boxer shorts and a singlet - slept on mattresses on a floor in the presbytery with the boy and his younger brother some time between June and September 1991.
He denied assaulting the boy.
Mr Abbott, then a federal parliament backbencher, told the court Fr Nestor was an upright and virtuous man who he had known since 1984.
''He was ... a beacon of humanity at seminary,'' Mr Abbott said.
Four other witnesses also testified to Fr Nestor's good character.
Fr Nestor was bailed pending an appeal and never served time behind bars.
In October 1997, Fr Nestor won his appeal in the Wollongong District Court.
District Court Judge Joe Phelan said Fr Nestor was of good character but should have been ''more prudent''.
''Inappropriate conduct does not prove that a criminal offence took place,'' the judge said.
However, the church never allowed the priest to return to ministry and about four years ago he was struck off the clergy list, or ''laicised'', by the Vatican.
It is understood Mr Abbott communicated with Fr Nestor twice after the court cases, but has had no contact with him for almost 15 years.
''In 1997, Mr Abbott provided a reference for Mr Nestor in an open court. He was subsequently acquitted by a District Court judge,'' a spokesman for Mr Abbott told AAP.
After the court case, a church professional standards inquiry found Fr Nestor should not be reappointed to the ministry.
The then-Wollongong Bishop, Philip Wilson, now Catholic Archbishop of Adelaide, advised Fr Nestor in writing ''significant additional material that I have received ... has been a cause of worry concerning your suitability for a further pastoral appointment in this diocese or any other''.
Fr Nestor appealed to the Vatican's Congregation for the Clergy, which decreed he be reinstated.
But in February 2001, the Wollongong diocese appealed and the decree was overturned.
The reasons for such decisions aren't made public and priests in the Wollongong diocese were informed of the result but not the detail.
However, West Wollongong parish priest Fr David O'Brien told AAP Fr Nestor had been ''forcefully laicised by the Vatican''.
Last year, Fr Nestor posted a message on the webpage of an internet group devoted to inactive or former Catholic priests.
''I was layicized (sic) on spurious grounds several years ago ... the life of a priest is not meant to be easy.''
AAP has been so far unable to contact the former priest, who is understood to have later worked in the NSW railways and lived in North America.