Mr Roach told the ABC he was an 18-year-old first-year seminarian when he met the recently ordained Father Walshe, then in his early 20s, at the Corpus Christi College in Clayton.

"One night he invited me up to his room, which was not uncommon," Mr Roach said.

"We had a fair bit of port to drink — I was very unfamiliar with drinking — and I woke up in his bed and he was abusing me.

"I left as quickly as I could, I was very confused, I didn't know what to do, what to think."

Mr Roach told the ABC that in two further encounters the following year there was an element of consent.

Father Walshe, a Mentone parish priest who was once Cardinal George Pell's housemate, denies he abused Mr Roach.

In a statement to the ABC, he said had engaged in "consensual conduct" with another adult in 1982, when he was emotionally vulnerable and sexually naive.

"My conduct was contrary to my religious beliefs. However it by no means constituted any form of abuse," the statement said.

Mr Roach, who now lives in the United States, told the ABC: "It can be denied by Walshe till hell freezes over, but the Church accepted my case."

He said he felt compelled to speak out after seeing Father Walshe give evidence at a hearing of the Royal Commission into Institutional Responses to Child Sex Abuse last week.

"Walshe cannot hold himself up as this paragon of virtue," he said.

"I think he is certainly a compromised witness."

Father Walshe's evidence at the royal commision was on his recollection of a phone call between Cardinal George Pell and child abuse survivor David Ridsdale in 1993.

Mr Ridsdale had earlier accused Cardinal Pell of trying to bribe him during the phone call, when he told him that his uncle, paedophile priest Gerald Ridsdale, had abused him.

Father Walshe, who was living with Cardinal Pell at the time, told the commission that immediately after the call Cardinal Pell had appeared upset and worried for the survivor.

"His demeanour," Father Walshe said, "was not that of a person that had been in a rude or angry conversation".

Counsel assisting the commission, Angus Stewart, SC, suggested Cardinal Pell's legal team had inserted a number of details into Father Walshe's statement.

"Father I put to you that you made up this statement to save your friend Cardinal Pell," Mr Stewart said.

Father Walshe replied: "I absolutely deny that."

Mr Roach told the ABC he left the seminary in 1983.

When he returned two years later, he had a meeting with the new rector, Cardinal Pell.

"In the course of the interview, he asked me why did I leave in the first place and I told him one of the principal reasons I left in the first place was that I had been abused by a priest," Mr Roach said.

"He said, 'I have got to ask you this, can you name the priest?' and I said 'sure, he is Father John Walshe', and he went, 'OK'."

The ABC reported Catholic Archbishop of Melbourne Denis Hart apologised to Mr Roach in 2012 for the "wrongs and hurt" he suffered at the hands of Father Walshe.

Mr Roach was offered the maximum compensation payment allowed under the church's complaints process.

However, Father Walshe was allowed to continue running the Mentone-Parkdale parish.

The ABC reported the lawyer appointed by the church to investigate the case, Peter O'Callaghan QC, wrote in his final report "there is no doubt that sexual abuse occurred".

He said this was because "a reasonable inference to be drawn is that J [the priest] had a degree of influence and control over the Seminarian".

Mr O'Callaghan defined sexual abuse as: "conduct of a sexual nature that is inconsistent with the public vows, integrity of the ministerial relationship, duties or professional responsibilities of church personnel."

The ABC reported the finding was not based on any legal interpretation of sexual abuse.