Papua New Guinea's new Government has told Australia it can't shut down the Manus Island detention centre and leave asylum seekers in PNG at the end of October.

Australia is trying to close the centre by October 31, something it said before PNG's recent national elections.

But PNG's newly-appointed Attorney-General Davis Steven summoned Australia's High Commissioner and told him the date had not been agreed with the PNG Government.

"I am the chief legal advisor to Government, I have not sighted a formal document that confirms that date has been mutually agreed," he said.

Mr Steven said Australia would not be allowed to simply close the detention centre and leave.

"The PNG Government is not going to allow a situation where Australia has withdrawn and leaves behind all these international fugitives who they expect us to carry on our steam," he said.

"It's not going to happen."

Mr Steven said the Australian Government has been asked to wait until the matter is discussed at the next meeting of the country's National Executive Council, its equivalent of Cabinet, before taking further action to close the centre.

Both the Australian and PNG governments have said the centre must be closed, after PNG Supreme Court ruled the asylum-seekers inside were being illegally detained.

But Mr Steven said the Australian Government had not explained why it had set the deadline of October 31 and was not saying what would happen to the men inside.

About 700 of them have been found to be refugees and are eligible for resettlement in the United States, depending on whether they make it through an "extreme vetting" process conducted by the US Government.

The men were sent to PNG under a Refugee Resettlement Agreement negotiated under then-prime minister Kevin Rudd in 2013.

Mr Steven said the agreement needed to be formally reviewed before any changes could be made.

He urged Australia to discuss the matter further with the PNG Government.

"I think we really need to come up with a clear understanding on the transition and how we manage the aftermath of the withdrawal," he said.

A spokesperson for Immigration Minister Peter Dutton said Australia remained committed to working with the country towards the closure of the centre.

"We look forward to further discussions with the PNG Government over coming days and weeks."