Scott Morrison has warned that child molesters, rapists and murderers could come to Australia if laws around evacuating sick asylum seekers from offshore detention are changed.
But Labor has rejected the suggestion as shameful scaremongering.
The prime minister declared "hundreds upon hundreds upon hundreds" of single men will be sent to Australia if Labor supports the changes in parliament next week.
Mr Morrison said the legislative amendments would shift the job of border protection away from the government to "any two doctors".
He claimed the bill would also stop the government from rejecting medical transfers to asylum seekers with criminal histories, despite supporters of the bill insisting these protections would remain in place.
"They may be a pedophile, they may be a rapist, they may be a murderer, and this bill will mean that we would just have to take them," Mr Morrison told reporters in Melbourne on Wednesday.
Mr Shorten told reporters in Townsville, where he was inspecting flood damage, the prime minister should be "ashamed of himself".
"The fact of the matter is people who have done those crimes don't get the refugee status unless the government's missed them when they've assessed them as refugees," he said.
"The idea that somehow because you're a Liberal you dislike those crimes more than if you're someone else, I just get sick of that moral superiority and finger pointing."
Labor supports the proposed legislation, which is expected to be voted on as early as next week.
The amendments, inspired by a push from Wentworth independent Kerryn Phelps, would change the process for medical transfers so that any two doctors could request a transfer.
The minister would then be required to review their case in 24 hours, and if they reject it, an independent health advice panel would review it.
Their advice could still be overruled by the minister on national security grounds, according to legal advice Labor received on Wednesday.
But Immigration Minister David Coleman says the definition of security in the bill does not cover people convicted of serious crimes, such as murder.
"The government's advice on this issue is extremely clear," he told AAP.
Under the current system, requests for medical transfers - made by doctors contracted by the government - are reviewed by a Department of Home Affairs committee.
But Mr Morrison also announced on Monday a new clinical panel will be established in the coming weeks, with the ability to ask immigration officials to review individual cases.
That comes as the Morrison government risks becoming the first in 90 years to lose a substantive vote in parliament, which would be widely seen as a loss of confidence.
But the prime minister insists the likely defeat will not trigger an early election.
"No, of course not, why would it be?" he said.
Australian Associated Press