Scott Morrison faces an uphill battle to unwind laws making it easier for asylum seekers to seek medical transfers.
The prime minister has nominated repealing the so-called "medevac bill" as one of his most pressing priorities.
However, it remains unlikely the repeal will gain Senate support, despite his re-election.
The roadblock could be costly.
The laws were passed by Labor and the independents against the Morrison government's wishes.
They give doctors more power to recommend the transfer of sick asylum seekers and refugees on Manus Island and Nauru for medical treatment.
Only one person was transferred to Australia under the laws during the first month, despite warnings from the government that hundreds would come within weeks.
The prime minister spent $185 million reopening the Christmas Island detention centre to deter asylum seekers from "gaming the system" under the new regime.
The budget revealed that if re-elected, the government would try to repeal the laws and close the centre by July.
"There is no form of the medevac bill which makes our borders stronger," Mr Morrison said earlier this year.
"The only thing that I would ever seek to do is repeal it in its entirety and the Australian people will have the opportunity to talk about that at the next election."
However, to repeal the medical transfer laws by the end of June, the government would need to recall the current Senate, which supports the legislation.
Otherwise, it would need to wait until the new Senate starts in July, missing the deadline.
But even then, the government would still struggle to repeal the laws, which Labor and the Greens support.
The coalition would need to convince five of six senators on the likely trimmed-down new crossbench.
However, the two Centre Alliance senators will not support unwinding the bill, leaving the government snookered.
"Centre Alliance supported the medevac bill in the last parliament and so we will not support it being repealed in the current one," Senator Stirling Griff told AAP on Monday.
The Morrison government initially budgeted $1.4 billion over four years to reopen Christmas Island, before slashing its forecasted spending through the plans outlined in the budget.
Australian Associated Press