The federal opposition has signalled the prime minister's backing of embattled Labor MP Craig Thomson may prompt a parliamentary vote of no confidence in the Gillard government.
Mr Thomson is under investigation by Fair Work Australia over allegations he misused union funds while he was national secretary of the Health Services Union (HSU).
The government will likely fall if Mr Thomson is forced to resign from parliament and Labor loses a by-election for his marginal NSW seat of Dobell.
The HSU is reportedly urging independent MP Andrew Wilkie, who has withdrawn support for the minority Labor government, to support a no-confidence motion if a case is proven against Mr Thomson.
Coalition frontbencher Christopher Pyne says the Thomson case will be the focus of opposition attention when parliament resumes in a fortnight.
"Julia Gillard has a lot of questions to answer," he told ABC Radio on Tuesday.
The parliament and public had a right to know about the prime minister's involvement in the three-year Fair Work Australia investigation, he said.
Mr Pyne linked the question of a no-confidence motion to the Thomson issue, but said it would only be used in "the most serious of circumstances".
The opposition says Ms Gillard has not adequately explained why, when she was workplace relations minister, her office contacted Fair Work Australia about the allegations against Mr Thomson.
Nor had the Labor MP made a statement to parliament about the issue, Mr Pyne said.
HSU national secretary Kathy Jackson has warned the investigation could be open to political manipulation and its findings pushed back to beyond the next election.
She told The Australian newspaper that if it was found that Mr Thomson had misused and misappropriated union funds, then that would be "an example of serious misconduct".
On that basis, Mr Wilkie should support a no-confidence motion.
Ms Jackson is supporting an opposition move that demands Fair Work Australia explain why its investigation into Mr Thomson has taken more than three years.
"Our members need answers," she said.
