Justice Terry Higgins presided over many a trial where an accused was acquitted during 23 years on the ACT Supreme Court bench, but nothing could compare to his critical judgment which has mandated 898 men – charged with nothing – must be released from the Australian-funded centre.

Justice Higgins, who spent a decade as chief justice, could have retreated to a comfortable life away from the public eye after his retirement in September 2013. But in March last year, aged 71, he became the most senior retired non-citizen judge to be appointed to the PNG bench.

Now he has been a key part of a decision which has led the PNG government to commit to closing the detention centre, and which could lead to compensation claims worth more than $1 billion from those illegally detained, one PNG refugee lawyer has estimated.

One of only two of the five judges to deliver reasons for his decisions, Justice Higgins ruled an amendment to the PNG constitution aimed at making the Australian-PNG deterrent arrangement an exception to the personal liberty right was invalid, as it breached technical requirements to change the foundation PNG law.

Leaders of the Canberra legal profession were not surprised to learn of the former ACT chief justice's decision to uphold the refugees and asylum seekers' constitutional right to personal liberty.

ACT Law Society president Martin Hockridge said as a solicitor, barrister and judge he had always been a forward thinker.

"He's very much been a champion of human rights," he said.

"There was a farewell dinner for three family law-based justices after the decision came out, and it was certainly noted [then] that one of our very own had made the decision, so there is a degree of pride from the Canberra legal community.

"The Law Society is generally against mandatory detention no matter what the circumstances are."

ACT Bar Association president Ken Archer, who appeared many times before Justice Higgins as both a prosecutor and later a defence barrister, said it was encouraging to see human rights being put "front and centre" in a legal decision.

"He's a good legal mind, and he always looked very closely at the interaction between individuals and the state," he said. "He often looked at the exercise of police powers."

On Thursday PNG immigration authorities said the men were no longer in detention, as they could now visit the main town on the island and were being bussed in and out of the centre each day.

For Justice Higgins, a final retirement must come by 2018, when he hits the maximum PNG judicial age of 75.