The Australian government's indefinite detention of refugees was unlawful because it gave politicians powers reserved for the courts.
The High Court on Tuesday revealed the reasoning behind its landmark November 8 ruling, which has so far prompted the release of more than 140 detainees from immigration detention.
The court also confirmed judges unanimously agreed indefinite immigration detention was unlawful, after earlier announcing a decision by a "majority".
Two of the judges wanted the court to make orders only when it published its reasons, and they also wanted more time to consider the matter before ultimately agreeing with their colleagues.
Judges in the decision found the indefinite detention of a Rohingya Muslim man from Myanmar, known as NZYQ, was "beyond the legislative power of the Commonwealth Parliament" while there was no realistic prospect he would be deported.
No country would settle the man after he raped a 10-year-old child and served time in prison.
He has since been released from immigration detention.
In its decision, the High Court overruled a 20-year precedent set by the 2004 case of Ahmed Al-Kateb, a stateless Palestinian man without a criminal record.
The Al-Kateb case found the Migration Act permitted indefinite detention, and said that detention wasn't in breach of the constitution's separation of powers.
The Rohingya man known as NZYQ argued the Migration Act was in breach of the constitution, given it grants courts "the exclusively judicial function of adjudging and punishing criminal guilt".
"The Court reopened and overruled the constitutional holding in Al-Kateb," judges said in a summary of the case published on Tuesday.
Judges decided sections of the Migration Act contravened the constitution because the Rohingya man's detention was not for a "legitimate and non-punitive purpose".
They pointed to another previous case, which found laws ordering someone's detention without a court order contravened the constitution unless they were for a justified and non-punitive purpose.
"In other words, detention is penal or punitive unless justified as otherwise," the judges said.
In the case of Al-Kateb, judges said immigration detention was justified and non-punitive given it meant a detainee could be deported later or prevented from entering the Australian community.
In the Rohingya man's matter, the judges said that approach was "an incomplete and, accordingly, inaccurate statement of the applicable principle" in circumstances where there was no real prospect of the plaintiff's removal from Australia.
However, they cautioned he could still be re-detained if the realistic prospect of his deportation arose, or if he was found to pose an unacceptable risk to the community as a child sex offender.
"Release from unlawful detention is not to be equated with a grant of a right to remain in Australia," the judges said.
They ruled the man's detention was unlawful as of May 30, about five months before the decision that set in motion his release.
National Justice Project chief executive and adjunct law professor George Newhouse said the High Court had limited politicians' right to lock people up indefinitely when there was no real prospect of deportation.
He noted that detainees released as a result of the decision could still be deported.
"On the face of it ... it is unlikely that NZYQ will be entitled to any compensation for wrongful imprisonment prior to 30 May 2023 given the findings of the High Court that it became unlawful after that date," Prof Newhouse said.
"Compensation issues will depend on the circumstances of each case."
The court's decision prompted the federal government to allocate $255 million toward enforcing strict visa conditions.
The cohort of released detainees has included convicted criminals.
Home Affairs Minister Clare O'Neil said the government was moving quickly to finalise a tough preventative detention regime before parliament rose.
Greens spokesman Nick McKim welcomed the court publishing its reasons and urged parliament to reflect on their detail.
