High court unanimously ruled indefinite detention was unlawful while backing preventive regime

Photograph: Lukas Coch/AAP

The high court has revealed its reasons for striking down indefinite immigration detention while suggesting a preventive detention regime could be legislated to re-detain non-citizens convicted of serious crimes.

On Tuesday, the court published its legal reasons in the case of NZYQ, revealing all seven justices decided earlier this month to overturn the precedent case of Al-Kateb which has underpinned Australia’s system of mandatory immigration detention since 2004.

On 8 November the court ordered that NZYQ, a 28 to 30-year-old stateless Rohingya man, be released because there was “no real prospect of his removal from Australia becoming practicable in the reasonably foreseeable future”.

In the reasons published on Tuesday, the judges stated that was when “the constitutionally permissible period of executive detention of an alien who has failed to obtain permission to remain in Australia” comes to an end.

But the judges warned that “release from unlawful detention is not to be equated with a grant of a right to remain in Australia”.

“Unless the plaintiff is granted such a right under the Migration Act the plaintiff remains vulnerable to removal under s 198,” they said.

“Nor would grant of that relief prevent detention of the plaintiff on some other applicable statutory basis, such as under a law providing for preventive detention of a child sex offender who presents an unacceptable risk of reoffending if released from custody.”

The opposition leader, Peter Dutton, has pressed the Albanese government to implement a preventive detention regime of that nature. The court’s comments appear to give that a judicial tick of approval.

The Albanese government has said it agrees in principle with such a move and the home affairs minister, Clare O’Neil, has said she would re-detain the people released as a result of the NZYQ decision if she could.

In joint reasons, the seven judges said the reasoning in Al-Kateb was “incomplete and, accordingly, inaccurate” in finding that detention was not punitive so long as its purpose was to make non-citizens available for deportation.

The judges noted that the agreed facts of the case included that on 30 May, NZYQ could not be removed from Australia, and also credited the government with “a correct and important concession” that they bore the onus of proving his detention was not punitive.

The judges rejected a narrower test put forward by the commonwealth that detention was unlawful only when there was “no real prospect” of removal, and a broader one proposed by the Human Rights Law Centre and the Kaldor Centre for International Refugee Law that it was illegal when it was “more probable than not” they would not be removed in the foreseeable future.

The Australian government had asked six countries to resettle NZYQ, who pleaded guilty to sexual intercourse with a 10-year-old and served a non-parole period of three years and four months before entering immigration detention. All but the US said no. It promised to take a “hard look” at the case.

The judges noted a home affairs department official’s evidence that it was impossible to predict if the US would say yes, concluding that although deportation to the US “remained a possibility” the government had failed to show it was “realistic” in the foreseeable future.

The decision caught the Albanese government by surprise, with internal advice and public statements by O’Neil indicating it had expected a result in early 2024.

It resulted in the release of 141 people from immigration detention so far – and a package of emergency legislation to impose electronic monitoring and curfews on those released.

The decision provides little guidance about how to treat difficult cases who remain in detention because of a lack of cooperation, noting that this was not a case where the plaintiff had “contributed to the frustration” of attempts to deport them.

In a statement, O’Neil and the immigration minister, Andrew Giles, said they noted the reasons for the decision that “the commonwealth strongly opposed”.

“Community safety remains the utmost priority of the government, which will continue to work with authorities to carefully consider the implications of these reasons and finalise rigorous and robust legislation,” they said in a statement.