The Department of Home Affairs has asked the Australian federal police to investigate how a confidential security agency briefing found its way into the Australian newspaper.

Shortly after Scott Morrison declared the proposed refugee medical evacuation amendments to be “stupid” legislation “written by people who haven’t got the faintest idea how this works”, parts of a confidential briefing based on Asio advice warning of its impact were published by News Corp.

The advice, which the government has declined to release, despite its front-page publication and use in political attacks, is understood to have been prepared by the nation’s spy agency and Australian Border Force for the home affairs department. It remains unknown if the reportedly “classified” advice refers to the original Kerryn Phelps bill, or the amendments, which strengthen ministerial discretion.

The Labor leader, Bill Shorten, has accepted a briefing from security agencies both about the practical implications of the crossbench bill, and about the impact of a separate proposal from the government setting up an independent medical review panel to vet medical transfers of asylum seekers from offshore detention.

Labor currently supports the crossbench bill, and moving from that position would trigger a backlash in the caucus, but Shorten made it clear this week that he could change that stance and support the government proposition.

The two architects of the crossbench bill, Phelps and the independent senator Tim Storer, have urged Labor to hold the line. Phelps said Labor and other parliamentarians should not “cave in” to Morrison’s “scare tactics and deliberate misinformation” about their proposal.

The government has been escalating attacks on the crossbench proposal all week, at least in part in an effort to avoid a defeat on the floor of the House when parliament returns next week. The last time a substantive vote was lost in the House was 1929, and an election was called the day after.

Peter Dutton went on the offensive on Thursday, first declaring Shorten had been briefed by agencies about the implications of the Phelps bill, then acknowledging he hadn’t, then declaring he should have been.

After adjusting the first incorrect statement, Dutton said the Labor leader needed to explain why “he couldn’t be bothered to turn up to a briefing on national security”.

Somebody gave a newspaper classified information Chris Bowen

The shadow treasurer, Chris Bowen, said Labor had “read in the newspaper that Scott Morrison was going to offer us a briefing”, with the article referring to “a letter expected to be sent from the prime minister to the leader of the opposition”.

“The newspaper had it before Bill Shorten did,” Bowen said. “That’s not how you have a sensible conversation.

“As I understand it we have now received the letter after we read about it – and we are considering what’s in that letter.”

On the leak of the reportedly classified briefing, Bowen called it “a very disturbing politicisation of our security agencies”.

“Somebody gave a newspaper classified information,” he said on Thursday.

“Now, there used to be a time when that was not the done thing. If somebody was caught giving classified briefings to a newspaper they had very serious consequences to pay. Under this government it appears that you are rewarded for it.”

The medical transfers bill is set to be dealt with in the House of Representatives when parliament resumes next week, after a frantic game of political cat and mouse to delay it reaching the chamber on the last sitting day last year.

After amendments were added to the bill to give the minister the discretion to reject medical transfers on national security grounds, Labor gave its support to the bill, which, with the majority of crossbenchers, gives it the numbers to pass the House, despite the government’s opposition.

Under the amendments, the minister would have to tell the parliament the reason for rejecting the transfer, which would then be further reviewed by an independent medical panel for assessment.

The government argues that would put too much power in the hands of doctors, at the expense of national security – although any evacuees would remain in detention in Australia, with the minister retaining ultimate control over who is released into the community.

In an attempt to head off the coming battle and a potential historic defeat on substantial legislation, the prime minister announced that the government would establish a medical review panel to look at transfer cases, but that the decision would remain with the minister.

The majority of crossbenchers have rejected that option, but Cathy McGowan, who indicated she would support the amended bill late last year, has said she will not make her final decision known until the bill comes to a vote.