Australia's detention of asylum seekers on Manus Island in Papua New Guinea is unconstitutional and illegal, a court has ruled.
Canberra has been criticised internationally for sending those who try to enter Australia by boat to seek asylum to processing centres on Manus or the small Pacific island of Nauru.
It refused to change its policy and Papua New Guinea's then opposition leader, Belden Namah, challenged the arrangement in court.
Now Papua New Guinea's Supreme Court has found that detaining asylum seekers on the island was "contrary to their constitutional right of personal liberty".
"The detention of the asylum seekers on Manus Island in Papua New Guinea ... is unconstitutional and illegal," it said in a 34-page ruling.
The court ordered the Australian and Papua New Guinean governments to "take all steps necessary to cease and prevent" the continued detention of the asylum seekers and transferees on Manus.
Australian Immigration Minister Peter Dutton said the court's decision "does not alter Australia's border protection policies - they remain unchanged".
"No one who attempts to travel to Australia illegally by boat will settle in Australia," he said in a statement.
Mr Dutton added: "Those in the Manus Island Regional Processing Centre found to be refugees are able to resettle in Papua New Guinea.
"Those found not to be refugees should return to their country of origin."
There are currently around 850 men held at the Manus detention centre.
Under a policy accepted by both sides of politics in Canberra, asylum seekers found to be genuine refugees are refused resettlement in Australia and are urged to return home or be resettled in PNG or Cambodia.
Australia argues it is saving migrants' lives in the long-term by not allowing entry and therefore preventing illegal people smuggling.
Human rights campaigners say the Manus detention centre should be shut.
"PNG's Supreme Court has recognised that detaining people who have committed no crime is wrong," said Elaine Pearson, director of Human Rights Watch in Australia.
"For these men, their only 'mistake' was to try to seek sanctuary in Australia - that doesn't deserve years in limbo locked up in a remote island prison.
"It's time for the Manus detention centre to be closed once and for all."