Law professors Bernadette McSherry, of Monash University, and Patrick Keyzer, of Bond University, compared government policies on the management of sex offenders in some Australian states, the US and Scotland in their book Sex Offenders and Preventive Detention: Politics, Policy and Practice.

Last month, Victoria's upper house introduced the Serious Sex Offenders (Detention and Supervision) Bill 2009, which would enable the Supreme Court to impose continued detention in prison of up to three years on serious sex offenders who had an "unacceptable" risk of re-offending. It would also allow supervision in the community for high-risk sex offenders, with cases reviewed annually.

Queensland, Western Australia and NSW have adopted similar laws, with the Victorian upper house to continue debate on the bill this week.

However, Professor McSherry challenged the cost-effectiveness and human-rights impact of preventive detention schemes for high-risk sex offenders.

"Two large studies have found that only about 13per cent of sex offenders re-offend within four to five years of release, yet state governments in Australia find it easier to spend hundreds of thousands of dollars to keep someone in prison rather than the tens of thousands it would take to put adequate support systems in place to manage them upon release."

Professor McSherry said there was a wealth of evidence that secure housing and support services outside prison reduced the re-offending rates of sex offenders.