A move to cut support for up to 400 asylum seekers in the Australian community could be blocked in the Senate and has been slammed by human rights lawyers and the opposition.
The Human Rights Law Centre and Asylum Seeker Resource Centre say about 100 people will be immediately affected by the Turnbull government's crackdown, but estimates the number could be as high as 400.
Greens leader Richard Di Natale said he was seeking advice on whether the Senate could block the instrument to create a new visa that would cut $200-a-fortnight payments and public housing for asylum seekers.
"We do call on members of the crossbench and the Labor Party to support us in doing everything we can to stop this unspeakably cruel act through the Senate," Mr Di Natale told reporters in Melbourne.
"If this is a disallowable instrument, it simply requires a majority of the Senate to stop it. So the question is for Bill Shorten and Labor - will you end this cruelty?"
Opposition Leader Bill Shorten said it was a new low for the government of Malcolm Turnbull, but made no mention of the Greens' more.
"Malcolm, this is not strong. This is cowardly and cruel. It's your weakest move yet," he said in a Facebook post.
Human Services Minister Alan Tudge could not confirm the precise number of asylum seekers at risk of being sent back to Nauru or Manus Island or their country of origin, but said there wouldn't be any further provision of taxpayer support in Australia.
"They've been prevented from working," Human Rights Law Centre executive director Hugh de Kretser told reporters in Melbourne.
"And now, completely out of the blue, with no notice whatsoever, they've been told tomorrow, you have no income we're taking all of your income away and in three weeks time we're taking your homes away."
Advocate Natasha Blucher said the asylum seekers were "very, very employable" and wanted to work, but with their history of trauma and the short notice, getting on their feet in "this time frame is absurd and it's impossible and it will end with children homeless."
Mr Tudge said the move was consistent with the principle that anybody who arrives by boat would not be settled in Australia.
"They will be settled elsewhere. That's what this is about," he said.
He did not think it was unreasonable to withdraw taxpayers support if they refuse to return back to Manus or Nauru.
Labor immigration spokesman Shayne Neumann said the Turnbull government had "sunk to a new low".
"By purposefully making these people destitute and homeless, the Turnbull government can only be exacerbating the health conditions which asylum seekers were originally transferred to Australia to be treated for," Mr Neumann said in a statement.
A Department of Immigration document said income support would cease from Monday, August 28 and a "final departure Bridging E Visa" would be issued giving asylum seekers three weeks to move out of government supported accommodation.
They will also be expected to sign the Code of Behaviour when released into the Australian community.