The Morrison government is exposed to a humiliating defeat in Parliament from a multi-party push to legislate faster medical treatment for refugees on Manus Island and Nauru, highlighting the Coalition's vulnerability on the last sitting day for the year.
Labor and the Greens have formed an alliance with key crossbench MPs to force an end-of-year clash on the issue after calls from the Australian Medical Association for urgent intervention to get children off Nauru.
The plan sets up a vote that could pass new laws on Thursday despite the government’s objections.
In a significant challenge to the government’s authority in Parliament, the alliance has gained a majority in the Senate and appears confident of securing the numbers in the House of Representatives to amend the law.
Prime Minister Scott Morrison said on Thursday morning the move was a "destructive, irresponsible and reckless move" on the part of the Labor Party and the crossbench.
Mr Morrison told Sydney radio station 2GB medical transfers would "basically completely crumble offshore processing in this country.....for one pathetic reason: to play games in the House of Representatives. They are weak as [the Labor Party and the crossbench], when it comes to border protection."
He said it was not "100 per cent clear" whether the government would lose the vote.
The government has been transferring children and some family members from Nauru to Australia but is under pressure to remove all children and allow faster movement for others when doctors recommend medical treatment in Australia.
One government source said there are currently 10 children on Nauru but four of them have been approved to go to the United States under a resettlement deal, leaving six. Almost all the 1,000 asylum seekers or refugees subject to the new bill are adults.
The new amendments aim to force the hand of ministers to allow transfers upon medical advice, using changes agreed by Labor, the Greens and Senate crossbenchers Tim Storer, Derryn Hinch, Rex Patrick and Stirling Griff.
The alliance in the Senate gained a narrow majority of 39 out of 76 votes for a motion on Wednesday that sets up a vote on the amendments on Thursday.
In an opportunistic tactic, the changes to the rules on medical treatment will be added to a separate migration bill put forward by the government, making it difficult for the Coalition to stop the changes in the upper house.
The vote is scheduled for 12.45pm in the Senate and could see the amendments put to the House of Representatives soon after Question Time on Thursday.
The plan aims to succeed where independent MP Kerryn Phelps struggled earlier in the week to legislate the urgent transfer of refugees or asylum seekers who are psychologically or physically ill, on the recommendation of two or more treating doctors.
While Dr Phelps required an absolute majority of 76 out of 150 votes in the lower house to legislate her plan, the alternative approach sets a lower bar for success.
Labor is aiming to secure the support of crossbenchers in the lower house including Dr Phelps, Adam Bandt from the Greens, Rebekha Sharkie from Centre Alliance, independents Cathy McGowan and Andrew Wilkie as well as former Liberal MP Julia Banks.
This would amount to 75 votes with all Labor MPs and all these crossbenchers voting in favour of the bill, enough to overcome any objections from the government.
The government has only had 75 seats in the lower house since former prime minister Malcolm Turnbull left Parliament, but only has 74 votes on the floor of the chamber. One of its members, Tony Smith, sits as the Speaker and does not vote unless there is a tie.
Immigration Minister David Coleman warned the changes could weaken border protection.
"Labor teaming up with the Greens would be the green light people smugglers have been waiting for," he said.
"The last time Labor joined with the Greens on border security, 50,000 people arrived illegally, 8000 children were placed in detention, and tragically 1200 people drowned at sea.
"A deal with the Greens would go against everything Labor has said over the past five years, proving again that Bill Shorten cannot be trusted."
Senator Storer said the amendments were an opportunity to provide certainty for people trapped on Nauru by putting an end to a "cruel and punitive regime" and giving more control to doctors rather than public servants and politicians.
"It will do so without jeopardising the security of Australia’s borders. The fact that the people smuggling trade did not resume after the announcement of the resettlement deal with the US shows that," he said.
"Time is running out... the situation on Nauru has reached critical point. Action is needed now."