A move to radically reduce the threshold for deciding to send asylum seekers back to possible danger will violate rights and endanger lives, leading refugee lawyer David Manne has warned.

Under sweeping changes introduced to Federal Parliament on Wednesday, those whose protection claims are rejected face return to their country unless it is decided they are ''more likely than not'' to suffer significant harm.

Immigration Minister Scott Morrison says the existing threshold, under which they are not returned if there is a ''real chance'' of them suffering harm, means they can stay as this risk is ''as low as 10 per cent''.

The new ''more likely than not'' test would mean there would have to be a ''greater than 50 per cent chance'' of a person suffering significant harm for them not to be returned, he said.

The change, covering those seeking protection under international treaties against torture and on civil and political rights, was one of many to toughen the process for seeking asylum.  It does not apply to those seeking protection under the refugee convention.

While Mr Morrison insists the new law is in line with the approach of Canada, Switzerland and the US, Mr Manne said it was contrary to accepted practice and could carry grave consequences and ''ultimately risk the lives of many''.

''What this does is propose a fundamental deviation from the well-established threshold for assessing someone's risk of facing life-threatening harm,'' he said.

Mr Manne said no case had been made for this and other changes that would ''downgrade due process and impose restrictions on fundamental rights''.

Mr Morrison said the government was committed to ensuring it abided by its international obligations. ''This is an acceptable position which is open to Australia under international law and reflects the government's interpretation of Australia's obligations.''

''This bill deserves the support of all parties. We need the tools to ensure public confidence in Australia's capacity to assess claims for asylum in the interests of this country, and against the interests of those who show bad faith.

''These changes uphold the importance of integrity, the establishment of identity, and increased efficiency in our protection processing system,'' he said.

But the changes shocked human rights lawyers.

''These amendments would allow the government to send people back to their country of origin even if there's a 49 per cent chance they will be killed or tortured,'' said Daniel Webb, director of the Human Rights Law Centre.

Greens spokeswoman Sarah Hanson-Young said it was a dangerous attack on vulnerable asylum seekers already in Australia.

''If you can't prove that you are more likely to be shot than not, you will be on your way home.''

Labor immigration spokesman Richard Marles said the changes were troubling. ''We would be extremely concerned if the government attempts to use complex legislation to sneak through shifting the goal posts on what determines refugee status,'' he said.

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