Those opposing the policy have introduced a bill to amend the Migration Act to allow the temporary transfer of these people to Australia for medical assessment.
The Australian Medical Association has backed the bill – but the government fears it could open a loophole that asylum seekers could exploit.
“We cannot have Australia’s borders determined by panels of medical professionals,” prime minister Scott Morrison told a news conference on Saturday.
Defence minister Christopher Pyne and immigration minister David Coleman also voiced their opposition to the bill.
Mr Pyne said the government would be forced to reopen a detention centre on remote Christmas Island if the bill was passed as most of the 1,000 asylum seekers in the camps would travel to Australia for medical assessment.
He said reopening the centre would cost 1.4bn Australian dollars (about £770m).
“They’ll be coming to Australia one way or the other saying that they have a need to because of ill health,” Mr Pyne said in an ABC television interview on Sunday.
Mr Coleman said the change would cause a return to the days when thousands of asylum seekers travelled to Indonesia and then paid smugglers to take them on to Australia by boat.
Many drowned when their boats got into trouble.
“We are talking about a catastrophic failure under the previous government where 1,200 people drowned at sea, where 50,000 people arrived and 8,000 children were forcibly placed into detention,” he said in a Sky News interview.
Tony Bartone, president of the Australian Medical Association, said in a statement that the government should set up “appropriate mechanisms” to allow the temporary transfer to Australia “for those in need of urgent care”.
Parliament could vote on the amendment when it resumes on Tuesday.
The ruling coalition could lose the vote as it is clinging to power with the support of seven independents.