The defeat of a High Court challenge to Western Australia's High Risk Serious Offenders Act will lead to further over-incarceration of Aboriginal people, a leading human rights and law expert says.

The case brought on by Peter Garlett challenged the constitutionality of the Act, which allows for extended detention beyond the end of a sentence and for the imposition of a wide range of restrictions after release.

Mr Garlett was sentenced to three and a half years imprisonment, including time already served since his arrest in 2017, for robbery, and was sentenced to a further five months for criminal damage while in prison.

He had been due for release in October 2021 but was held in prison until April this year under a detention order sought by the WA Government.

Law academic and human rights expert Hannah McGlade said the case highlighted a failure of Australian law.

"We do not have constitutionally entrenched human rights in this country. We don't have a charter of human rights," she said.

"People look for implied rights in the constitution, the conservative High Court has whittled down the previous recognition in the Kable case, on which we were relying."

Ms McGlade praised the two judges who backed the challenge.

Justice Jacqueline Gleeson, one of five judges to rule against the challenge, said there was no part of the Act which compromised the WA Supreme Court's independence

"To the contrary... the HRSO Act requires the Western Australian executive to make an application to the Court, in accordance with the legislation," she said.

"The application is then assessed by the court in accordance with the legislation."

WA Attorney-General John Quigley said the High Court's decision kept West Australians safe.

"Our laws dealing with the most violent and serious offenders are an expansion of the earlier dangerous sexual offender regime," he said.

"They are based on the premise that offenders who commit serious and violent crimes, and continue to pose an unacceptable risk of violent reoffending, should remain behind bars or under supervision in the community until such time as they are no longer an unacceptable risk to the community.

"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."

Mr Garlett was freed in April after his lawyers took his case to the Supreme Court of WA.

In that case, Judge Larissa Strk rejected the State's application to keep the man detained indefinitely.

After his release, Mr Garlett told National Indigenous Times the prison system was breaking people.

"Being out of that cell just one hour a day, no recreation because of COVID, not even being able to play footy, always locked away like a dog, it is hard," he said.

"A lot of young boys in there who don't know what's going on, they aren't equipped to get a lawyer and to take the system on.

"They need to help young people, young Indigenous people, live their lives - get them out so they can be role models to their family and their children."

Mr Garlett said more Indigenous staff were needed to fix the prison system.

He said at the time he was happy and relieved to be reunited with his loved ones.

Derek Charles Ryan, who was placed on curfews, 24-hour monitoring, travel bans and geographical limits under the Act after serving his time, said he was devastated by the decision.

"I was relying on justice and common sense - this decision is unjust," he said.

Roe Legal principal Paul Sheiner, who represented Mr Garlett, said he is concerned about the impact of the finding on prisoner well-being and is considering a complaint to the United Nations Human Rights Committee.

Two of the seven High Court judges, Justice Michelle Gordon and Justice Stephen Gagelar, rejected the majority ruling issued Wednesday.

In her judgement, Justice Gordon said describing the High Risk Serious Offenders Act as "preventive justice" is a misnomer.

"The premise of the HRSO Act is that those who have offended may offend again... Labelling the HRSO Act scheme, and the role of the Supreme Court, as 'preventive justice' is a misnomer. It is not justice," she said.

"The HRSO Act scheme, at least in its operation with respect to robbery, is contrary to Chapter III (of the Australian Constitution) and undermines the two key rationales - or constitutional values - underpinning Chapter III's strict separation of Commonwealth judicial power from executive and legislative power: first, the historical judicial protection of liberty against incursions by the legislature or the Executive; and second, the protection of the independence and impartiality of the judiciary so as to ensure that the judiciary can operate effectively as a check on legislative and executive power."