Labor is calling for an independent inquiry into the government's decision to appoint Nigel Hadgkiss to head the building industry watchdog, despite legal concerns.
Mr Hadgkiss resigned from his $426,000 job at the Australian Building and Construction Commission last week after he admitted to breaching workplace laws during his earlier tenure as head of the Fair Work Commission's building industry office.
Mr Hadgkiss contravened the Fair Work Act by instructing staff to not publish legal changes to right-of-entry rules for unions. He made the admission as a result of legal action brought by his chief target, the Construction, Forestry, Mining and Energy Union.
It was an embarrassing development for the Turnbull government, which used the ABCC as the catalyst for last year's double dissolution election.
The government passed its laws to resurrect the Howard-era watchdog in December, four months after the CFMEU launched its legal proceedings.
Employment Minister Michaelia Cash said she learnt of the allegations against Mr Hadgkiss in October but proceeded with Mr Hadgkiss' appointment.
Opposition employment spokesman Brendan O'Connor has written to Prime Minister Malcolm Turnbull calling for a independent inquiry, saying Senator Cash had failed to answer key questions about the issue.
"This refusal gives rise to the inference that proper process was not followed by the minister and that the Ministerial Standards were breached," Mr O'Connor says in the letter.
The first official complaint against Mr Hadgkiss was made on July 26, 2016. He took the offending material down on July 28, 2016, and legal proceedings were launched on August 19.
Mr O'Connor said it could not "reasonably be accepted that the minister was not aware of the legal proceedings against Mr Hadgkiss" earlier than October. Such as assertion was a "concession of negligence for failing to make appropriate inquires".
A spokesman for Michaelia Cash said: "There was no cabinet appointment process for the position because, when the ABCC legislation passed the Parliament, the legislation prescribed that the head of the FWBC automatically became the head of the ABCC."
ACTU president Ged Kearney said on Sunday that Mr Hadgkiss' conduct strengthened the case for a national corruption commission.
But Australian Chamber of Commerce and Industry chief executive James Pearson said Mr Hadgkiss, a distinguished and long-serving public servant, had resigned, unlike many CFMEU officials accused or even convicted of wrongdoing.
"Who have to be taken to court at considerable public expense, who are regularly found guilty, who are subject to fines totalling now in the tens of millions of dollars," Mr Pearson told ABC television.
Mr O'Connor conceded the record fines of $2.4 million imposed on the CFMEU last week might not have been as stiff under a Labor government.
The Federal Court imposed the penalties against the militant union last week for an unlawful blockade at Lendlease's Barangaroo site in Sydney.