The secretary of the Department of Justice, Greg Wilson, is challenging a Supreme Court decision to revoke a decade-long supervision order against Robin Fletcher, who previously served eight years in prison for sexually abusing two teenage girls in the 1990s.

The order restricts him to living at a particular address, which he can only leave under strict conditions. He cannot move from that address without the Adult Parole Board's approval.

Justice Phillip Priest last week ruled that Fletcher, 60, posed no greater risk of sexual reoffending "than the average sex offender released into the community". He said there was a "low" risk he would reoffend in the same way, based on psychological evidence, the fact he is now legally blind, his age and his physical weakness.

Fletcher pleaded guilty to performing violent sexual acts on two 15-year-old girls when he was working as an alcohol abuse and sexual guidance counsellor. One of his victims later took her own life.

He maintains that his crimes - including whipping, sexually assaulting and prostituting the teenagers - related to his "Wiccan" religious beliefs, but has told forensic psychologists he now recognises such behaviour is illegal.

David Grace, QC, for the secretary, told the Court of Appeal on Thursday that the judge's assessment of Fletcher's risk was incorrect because it was only based on the results of one of the psychological tests conducted on him, which had not changed since the 1990s.

While Mr Grace conceded that he was unlikely to be able to become a counsellor again, he said his religious beliefs were "intractable" and that he still attended a church in Melbourne.

Given it was well known that child sexual abuse caused "catastrophic" harm to victims, even a "moderate risk" of such harm was unacceptable and justified keeping Fletcher under supervision, he said.

If the court upheld the judge's ruling, he continued, it would make it difficult to renew similar supervision orders on other child sexual offenders because "it would place an impossible task on the secretary to differentiate between harm caused (by) Fletcher's offending and other victims of this horrible behaviour."

Justice Robert Redlich disagreed, saying that if he was right, the court could never free a child sexual offender from a supervision order.

Justice Priest last week ruled that while any future child sexual abuse by Fletcher would be "debilitating" to his potential victims, the extent of harm caused depended on the person. The judge said the consequences of such crimes would be "no greater than is generally experienced by adolescent female victims of aberrant sexual activity perpetrated by an adult male."

Defence counsel Christopher Carr said that his client did not share the same characteristics as persistent child sexual offenders over 60 who typically had more victims, and were sentenced more often for their crimes.

President of the Court of Appeal Justice Chris Maxwell, Justice Robert Redlich and Justice David Beach reserved their decision, and ordered that Fletcher remain under supervision until it was handed down.

Justice Maxwell said they would deliver a ruling "as a matter of urgency", noting that his liberty was at stake.