Scott Morrison is facing another potential defeat on a vote his government opposes when parliament returns on Monday.
The coalition lost a vote on asylum seeker medical transfers in the lower house on Tuesday, forcing the government to change the law against its wishes.
Now a long-buried bill to make it easier for small businesses to legally challenge big businesses could be the next problem for the prime minister.
The bill, which has been on hold since 2017, would allow small businesses to apply in court to escape paying costs against big businesses even if they lose a court case.
Shadow assistant treasurer Andrew Leigh said the government was now worried the Nationals would cross the floor to support it in the lower house.
"The Nats love this proposal," he told AAP on Friday.
"The Liberals have been holding the Nats back from supporting it."
When the proposal came up in the Senate again on Thursday, Finance Minister Mathias Cormann withdrew the government's opposition to it when he realised he did not have the numbers to vote it down.
Mr Morrison will also deal with Labor's push for a disability royal commission - if he doesn't call one himself over the weekend.
The coalition's "big stick" energy reforms will also be under scrutiny, with the Nationals expecting it to be voted on before the May election.
Senate estimates hearings will start for the first time in 2019 and government departments are already facing serious scrutiny.
Peter Dutton's Home Affairs department faces questions about a contract for Paladin, a company looking after asylum seekers on Manus Island.
The company was reportedly given a $400 million tender for which it was the only bidder, despite not having enough money to start the contract and its founder having a history of bad debts.
And the same department will face fierce scrutiny over its role in Hakeem al-Araibi's detention in Thailand.
The 25-year-old refugee footballer was on his honeymoon when Bahrain got a wrongly-issued Interpol red notice against his name and he was detained upon landing in Bangkok.
Home Affairs will face questions about how Bahraini authorities knew al-Araibi was travelling, and whether the Australian Federal Police played a role in tipping them off.
The Department of Parliamentary Services will also be quizzed over Senator Brian Burston's flight with One Nation staffer James Ashby after a dinner in the Great Hall on Wednesday.
Senator Burston later admitted to swiping blood on Senator Pauline Hanson's door, while Mr Ashby had his parliamentary pass revoked.
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