There is a category of serial sex offenders who may never be rehabilitated and who need supervision well into their old age to protect the community, according to forensic psychiatrist Robert Kaplan.
Approached by the Mercury on Tuesday, Dr Kaplan said he could not comment on the case of Bulli rapist Terry John Williamson, but said there had been reports of paedophiles in their 70s and 80s, with very poor mobility, who had re-offended despite having undergone intense treatment programs and chemical castration.
He said these men had claimed they no longer had the urge or desire to molest children, but when presented with an opportunity had done so anyway.
Williamson is now 44. At his sentencing in 1991 the Supreme Court heard he had an extremely high libido and found it difficult to relate to or attract women. He said frequent masturbation alone had not been enough to suppress his overwhelming sexual appetite.
As part of interim supervision orders handed down by the Supreme Court, Williamson is still on anti-libidinal drugs and has said he no longer has the sexual urges he did when he committed his crimes.
Chemical castration is the administration of female hormones to lower an offender's deviant sexual fantasies and arousal.
Dr Kaplan said the drugs combined with a treatment program had a good response for mild to moderate sexual offenders. Rehabilitation for high-risk sexual predators could be more difficult.
"You have to get to a point where you can remove sex drive and change behaviour. Then you have to make sure the person can maintain it. Medication alone won't get results for some offenders."
He said that to change deviant behaviour a sexual predator needed to change their whole lifestyle which usually revolved around creating opportunities for themselves to molest and attack their victims.
"It can be very, very difficult," he said. "Some offenders will only be complying with the programs because they are in trouble with the law."
Williamson's reign of terror lasted 10 months. He kidnapped, raped and sexually assaulted 11 victims, including children as young as five, often in their own home. At the time he was 20.
While not referring in any way to Williamson's case, Dr Kaplan said there were some serial rapists whose criminal activity led them to becoming serial killers. In Australia a classic example was backpacker murderer Ivan Milat.