The Abbott government's plan to reduce the threshold for assessing whether people face torture or degrading treatment if they are returned to their countries of origin faces certain defeat in the Senate.
But the fate of several other provisions that will make it more difficult for thousands of asylum seekers to press their claims for refugee status remains uncertain.
Labor, the Greens and several crossbenchers plan to vote down a new framework for assessing the claims of those who are not refugees, but seek "complementary protection" under other international treaties, including the convention against torture and other cruel, inhuman or degrading treatment.
Critics including crossbenchers Ricky Muir and Nick Xenophon, fear the changes raise "the real prospect of returning people to persecution or other forms of life-threatening harm".
If passed, the legislation would apply a "more likely than not" threshold that would only protect those who face more than a 50 per cent chance of suffering significant harm if returned to their home countries.
The existing framework applies the same threshold for complementary protection as for assessing refugee claims: that there is a "real chance" of the person facing harm if returned.
Several senators have expressed concern that the lower threshold could result in women facing honour killings or female genital mutilation.
Labor has vowed to oppose the lower threshold and will move amendments to several other provisions. These include one that would refuse protection visas to those who do not provide evidence of their identity without a valid explanation. Another change requires asylum seekers to specify all arguments to support their claims from the outset.
Greens senator Sarah Hanson-Young says the legislation will deny refugee protection to those who are thought to have, or who actually have, provided false identity, citizenship or nationality documents.
She says the change "ignores the realities of seeking asylum and goes against the basic principles of the refugee convention".
Liberal senator Cory Bernardi defended the legislation on national security grounds, telling the Senate: "If you look at some of the issues we are facing in this country today, whether it is terrorist activity, insurgent activity, extremist activity or general crime, you will find there are elements of this that can be traced back to a disorderly migration program.
"To say that is not being racist or anything like that; it is simply stating some facts and some truths."