A law in Western Australia allowing the state to indefinitely detain "high risk" offenders is constitutionally valid, the High Court has ruled.
Noongar man Peter Robert Garlett, 28, challenged WA's High Risk Serious Offenders legislation after being detained beyond his prison sentence for aggravated robbery.
Garlett had stolen a necklace and $20 in cash from two victims at their Perth home while pretending to be armed and threatening violence in the company of others.
The High Risk Serious Offenders legislation, introduced by the McGowan Labor government and passed through state parliament in 2020, targets offenders deemed likely to pose an unacceptable risk to the community.
It hands judges the power to order the continuing detention or community supervision of violent offenders who are nearing their release date.
Lawyers for Garlett argued the legislation was constitutionally invalid because it impaired the integrity of the WA Supreme Court, by requiring judges to exercise punitive and non-judicial powers.
But a majority of High Court judges on Wednesday rejected the appeal, finding that the scheme was protective rather than punitive.
"The HRSO Act does not require the court to give effect to any decision of the legislature or the executive government," Chief Justice Susan Kiefel, Justice Patrick Keane and Justice Simon Steward wrote.
"The performance by the court of this function proceeds by processes which are familiar aspects of the exercise of judicial power."
In a dissenting opinion, Justice Michelle Gordon argued the law went too far with its inclusion of crimes such as robbery.
There was a risk of such regimes becoming the norm, she warned.
"The HRSO Act goes beyond the exceptional," she wrote.
"The judiciary is left to perform the executive function of preventing a wide range of crimes one offender at a time ... without identifying any underlying justification of the kind that might be identified with respect to sexual offending or terrorism."
Garlett was convicted in 2019 and sentenced to three years and six months' imprisonment, having been held in custody since his arrest in 2017.
He had been due for release in October last year but the Supreme Court granted an interim detention order following an application from the State Solicitors Office.
Garlett was subsequently freed after lawyers took his case to the High Court.
The Supreme Court last year heard Garlett had spent long periods in custody, mostly for offences committed as a juvenile.
He was placed in foster care at the age of seven after being exposed to domestic violence and drug and alcohol abuse.
WA Attorney-General John Quigley welcomed the High Court's ruling, saying offenders who posed an unacceptable risk of violent reoffending should remain imprisoned or under supervision until that was no longer the case.
"Western Australians have the right to feel safe in their communities and these laws enable courts to keep the most dangerous criminals behind bars, in line with the community's expectations," he said in a statement.
National Justice Project principal solicitor George Newhouse said the scheme was being used to "trap Aboriginal people in the criminal justice system even after they've done their time".
"Instead of weighing them down with a ball and chain, the WA Government should provide people leaving the criminal justice system with the support they need to integrate back into the community," he said on Wednesday.