Manus detainees won't come here: Dutton

Immigration Minister Peter Dutton insists Australia's border protection policies will remain unchanged despite a Papua New Guinea Supreme Court ruling that detention of asylum seekers on Manus Island is illegal.

"People who have attempted to come illegally by boat and are now in the Manus facility will not be settled in Australia," he said in a statement.

The court ruled on Tuesday that detaining asylum seekers on Manus Island is in breach of the constitution.

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

Mr Dutton said Australia was not a party to the legal proceedings, brought by the PNG opposition leader Belden Namah.

"It does not alter Australia's border protection policies - they remain unchanged."

Mr Dutton said the agreement with PNG to establish the Manus Island centre was negotiated by the Labor government.

He said those at the centre found to be refugees were able to resettle in PNG while those found not to be refugees should return to their country of origin.

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

"It effectively means both governments must take steps to effectively shut down the Manus Island detention centre," he told Sky News.

"If it becomes apparent to us, to my client, within a reasonable period of time that no necessary steps have been taken by the authorities ... we're going to be seeing them back in court immediately."

Opposition immigration spokesman Richard Marles said Mr Dutton should immediately travel to PNG to hold urgent talks with the government.

He warned the decision and the government's response to it would be monitored by people smugglers.

"Labor is seeking an assurance from the government that it has a contingency plan to deal with today's ruling," he said.

He said the Manus facility was established as a circuit breaker and was "never intended to serve as a punitive place of indefinite detention".

"Mr Dutton and his predecessor Scott Morrison failed to properly engage with the government of PNG to ensure processing was occurring in a timely manner.

"They have also both failed in securing a lasting, third country resettlement to resolve the future for the people on Manus and Nauru."

Greens immigration spokeswoman Sarah Hanson-Young said it was time to close the centre and bring the asylum seekers to Australia.

She said the end had finally come for the government's "cruel and inhumane" detention camps.

"This is an important decision - it shows the Australian government has been illegally detaining refugees on Manus Island for years.

"Malcolm Turnbull has to act. He needs to allow the people there to be brought to Australia so they can have their claims assessed and be integrated into the community."