The federal government is concerned that asylum seekers who leave Manus Island or Nauru for medical treatment may never go back due to a legal loophole.
The government is looking at using "unusual powers" to close what it says is a loophole that would prevent asylum seekers transferred to Australia for medical treatment from being sent back to Manus Island and Nauru.
Attorney-General Christian Porter has blamed poor drafting of medical evacuation laws passed last week against the government's will for the loophole.
"There is no lawful authority for the government to return medical transferees once they come to Australia and they're detained on Christmas Island," Mr Porter told reporters on Thursday.
However, Labor maintains nothing has changed.
Labor's immigration spokesperson Shayne Neumann said the new laws had not changed the returns system.
"If a person is transferred to Australia for temporary medical treatment, we will return them to Manus or Nauru once doctors advise they have completed medical treatment," he said in a statement.
"This is a desperate distraction from a government trying anything to hide from its scandals and from bowing down to the big banks."
The government is refusing to release its legal advice, instead committing to provide a summary of the solicitor-general's opinion.
Mr Porter said changes to medical evacuation rules had given asylum seekers a one-way ticket to leave offshore processing.
"We're looking at whether there might be any other powers that we might be allowed to rely on rather than the ones that are traditionally relied on," Mr Porter told reporters on Thursday.
Mr Porter acknowledged that "very few" of the 900 asylum seekers already transferred to Australia for medical treatment from Manus Island and Nauru had returned.
"The overwhelming majority contest our lawful ability to send them back and they do that in the Federal Court," he said.
"The situation we now face [is] we don't have any authority at all, there'd be nothing to argue about in the Federal Court."
Under the new regime, two doctors can recommend asylum seekers currently on Manus Island or Nauru be transferred to Australian territory for treatment.
The government intends to send people to the reopened Christmas Island detention centre, off Western Australia, rather than the mainland.
The attorney-general said the government was "scrambling" to find a way to close the loophole but was yet to resolve the issue.
"We are doing our best to make a terrible law work as best we can," Mr Porter said.
Under the law, the home affairs minister has 72 hours to decide whether or not to agree to a medical transfer.
If the minister rejects the transfer, the decision may be reviewed by a medical panel, which can recommend it goes ahead.