Warning Aust govt could be sued over Manus

Lawyers are warning the Australian government could be sued for false imprisonment after a Papua New Guinea Supreme Court ruling deeming detention of asylum seekers on Manus Island illegal.

The court ruled on Tuesday that the detention of asylum seekers on Manus Island is unconstitutional and illegal.

It also ruled that a constitutional amendment made by the PNG government last year to legitimise the arrangement with the Australian government is also unconstitutional and invalid.

"Both the Australian and PNG governments shall forthwith take all steps necessary to cease and prevent the continued unconstitutional and illegal detention of the asylum seekers," the judgment says.

Immigration Minister Peter Dutton insists Australia's border protection policies will not change, and asylum seekers on Manus Island will not be brought to Australia.

"If people seek to come illegally by boat to our country, whether they're on Manus, they're on Nauru or they're negotiating with people smugglers now, they will not be settled permanently in this country," he told Sky News.

"We will not retreat from what has been a tough but fair policy."

But the Australian Lawyers Alliance says the decision strengthens claims that Australia breached its duty of care for detainees.

Alliance spokesman Greg Barns says the ruling is consistent with international law, which states indefinite detention is unlawful.

"The ruling makes it highly likely that asylum seekers can successfully make claims for damages for false imprisonment from the time they entered detention on Manus Island," he said.

"If Australia ignores the decision then it is contradicting its oft-stated claim that Manus Island detention is a matter for PNG jurisdiction."

Mr Dutton says Australia was not a party to the legal proceedings brought by PNG opposition leader Belden Namah, insisting the ruling is a matter for the PNG government.

He said the Australian government had contingency plans in place for a "number of outcomes in a number of court cases" and would work with the PNG government to send asylum seekers back to their country of origin.

The government is also working with countries in the region on third country settlement options, he said.

Loani Henao, legal counsel for Mr Namah, says the decision means the Manus Island detention centre must be shut down.

Labor is demanding Mr Dutton immediately travel to PNG for urgent talks with the government while the Greens want the asylum seekers brought to Australia.