Prime Minister Malcolm Turnbull has vowed the government's hardline border stance is unshakable after the High Court ruled offshore detention was lawful, but stopped short of saying when more than 250 asylum seekers will be flown to Nauru.

In a 6-1 verdict, the court on Wednesday ruled that the federal government has the power under the constitution to detain people in other countries, clearing the way for the return of more than 220 asylum seekers to Nauru, along with as 37 babies born in Australia.

Immigration Minister Peter Dutton had signalled the government's intention to move quickly, declaring last month that the transfer would leave only seven children in mainland detention, but Mr Turnbull is under pressure from church and advocacy groups and some Liberal MPs to be cautious.

Victorian backbencher Russell Broadbent appealed to the Prime Minister to gradually transfer those on Nauru and Manus Island to Australia - while continuing with the policies that have stopped the boats: boat turn-backs and the policy of processing any future arrivals on Nauru and Manus Island.

But Mr Broadbent warned that any move to transfer the children, and women who say they are victims of sexual assault, back to Nauru would prompt a backlash against the government, especially in Melbourne. "I don't think Malcolm wants that," he told Fairfax Media.

The Australian Human Rights Commission will also release a report on Thursday that President Gillian Triggs says will represent a powerful case for the government not to go ahead with the returns. The report is the findings of medical experts who assessed children at the Wickham Point centre near Darwin who face being returned. Professor Triggs says the clinicians had never confronted such traumatised children, and the prospect of return was aggravating their condition.

In question time on Wednesday, Mr Turnbull reiterated the government's resolve to deter asylum seekers and ensure "this pernicious, criminal trade of people smuggling cannot succeed", declaring: "The line has to be drawn somewhere and it is drawn at our border". But Mr Turnbull added that the government would consider the decision "and its implications carefully".

Mr Dutton later affirmed that those brought to Australia for medical treatment would be returned once there was no need for that assistance, hinting that family members of those being treated who accompanied them to Australia would be the first to be returned. But he gave no timetable for returns.

In a statement, Labor's Immigration spokesman, Richard Marles, did not address the key issue of whether asylum seekers should be saved from returning to Nauru, but called on the government to immediately find ways to resettle refugees in third countries. He later signalled his expectation that the group would be returned to Nauru.

"People have been left to languish in processing centres without any certainty for their future ... without an urgent resolution, this government will be doing enormous damage to this refugee population," he said in the statement.

Following the verdict, Human Rights Law Centre legal advocacy director Daniel Webb, whose organisation brought the case to court, said: "Legality is one thing. The morality is another.

"It is fundamentally wrong to condemn these people to a life in limbo on a tiny island," he said.

"The stroke of a pen is all that it would take our Prime Minister or our Immigration Minister to do the decent thing and let these families stay."

The test case was run on behalf of a Bangladeshi woman who was brought to Australia from Nauru in August 2014 for medical treatment. Supporters said the woman, who has a baby daughter, was "terrified" of returning to Nauru, where asylum seekers say they have suffered physical and sexual abuse, poor health care and inadequate living conditions.

The woman argued that the Commonwealth's conduct, including restraining her liberty and entering into contracts to allow her detention, was not authorised by any valid Australian law.

The court ruled that the woman was not entitled to declare that her past detention was unlawful. It said the detention regime was authorised under the constitution and other laws.

Refugee and Immigration Legal Centre executive director David Manne said the court considered the legal aspects of offshore detention, but not whether it was "good and proper policy".

"There now must be urgent consideration of how to end the suffering of the cruel limbo on Nauru. It can't and mustn't go on like this," he said.

"On any measure, whether it be moral, human, financial or practical, or indeed in light of our international law obligations, the policy is cruel, inhuman and unsustainable."

Professor Triggs said the court did not judge whether offshore detention met international obligations to "protect the safety and wellbeing of all people under our jurisdiction, including people seeking asylum."

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