The Manus Island detention centre deal between Australia and Papua New Guinea will not be directly threatened if the PNG Supreme Court rules the government of Peter O'Neill is illegitimate, a constitutional lawyer says.
The government of PNG and Australia signed a memorandum of understanding two weeks ago to reopen the centre, amid an ongoing Supreme Court case into the legality of Mr O'Neill's elevation to the top job on August 2.
Former government lawyer John Nonggorr told AAP the decisions of Mr O'Neill's government would still be considered legitimate, even if the court rules his government isn't.
"All decisions made by such a regime are presumed legitimate," he said. "It's not just a decision of asylum entries into Australia. It is financial commitments, appointment of officials, it's all those range of decisions.
"In order for smooth public administration, there's a legal principle in PNG that everything's legitimate until proven otherwise."
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National Capital District governor Powes Parkop has previously said he would consider challenging the Manus deal unless his demands for the treatment of asylum seekers were met.
The governor, a human rights lawyer, said that under section 42 of the PNG constitution, locking up detainees without charge would be illegal. "If they are locked up, it will be illegal," he said.
"The law here is very clear... a person cannot be locked up without charge".
The Supreme Court challenge into the legitimacy of Mr O'Neill's government continues on Thursday morning.
