Construction union boss John Setka has been referred to parliament's powerful privileges committee over alleged threats to senators.
Centre Alliance's Rex Patrick was on Thursday backed by the major parties to refer Mr Setka to the committee to see if he intimidated a group of crossbench senators over union-busting legislation.
The Greens opposed the referral, with leader Richard Di Natale warning of a "slippery slope" if politicians abused the powerful committee to oppose campaigning on issues.
Senator Patrick, his partyroom colleague Stirling Griff and independent Jacqui Lambie initiated the probe to investigate a possible contempt of parliament, which would result in possible fines and jail time.
The trio claim they were threatened when Mr Setka told a shop stewards meeting they would face abuse in the streets if they supported a bill making it easier to deregister unions and ban officials.
The matter has also been referred to the Australian Federal Police.
Mr Setka's construction union has welcomed the opportunity to appear before the committee.
"When politicians contemplate supporting draconian industrial relations laws that will threaten workers' rights - our members expect us to stand up and campaign against them," the CFMMEU said in a statement.
"The suggestion that campaigning against legislation which undermines the human rights of workers and their representatives is in any way in contempt of the Senate is anti-democratic and sets a dangerous precedent."
In a leaked recording, Mr Setka said the senators would "wear the consequences" of their decision, and they could expect to be abused on the street in 20 years' time.
"Someone is going to point the finger and say, 'There's them f***ers that voted for that bill that f***ed up not just construction workers but all workers in Australia'," Mr Setka said in the meeting last week.
The union described the recordings as illegally obtained, suggesting its use raised serious ethical questions.
Industrial Relations Minister Christian Porter told a business forum the laws were necessary because the current system was "totally insufficient" to change the behaviour of unions such as the CFMMEU and officials such as Mr Setka.
"The CFMMEU's record is truly and astonishingly appalling - 2166 separate breaches and contraventions of the law in the past 15 years - an average of almost three per week," he said.
"Over $16.5 million in court ordered penalties - and that's just for breaches of industrial law, it doesn't include those more serious criminal offences that the organisation and its officials have been convicted of in recent times, including of course drug dealing."
Australian Associated Press