The Senate has again rejected a bill for greater oversight of unions, setting up a double dissolution trigger for the Federal Government.

In a late-night vote, the legislation was defeated 34 votes to 33.

The Coalition wanted to set up a Registered Organisations Commission to regulate unions, a move Labor has been fighting.

If passed, the Fair Work (Registered Organisations) Amendment Bill would have imposed the same disclosure and transparency obligations on union officials as company directors.

The bill also increased civil penalties and introduced criminal penalties for union leaders involved in fraud.

In making his final pitch to the Senate, Employment Minister Eric Abetz said unions and companies should be held to the same standard.

"Why should a corrupt union official who has ripped off hundreds of thousands of dollars from a union only be liable for a fine of $10,800, when for the same corrupt conduct a company director would be liable for five years' imprisonment or a fine for $360,000 for ripping off shareholders?" he said.

"What is the material difference? What is the moral difference?

"There should be no material difference between the penalties."

It was the second time the Coalition's bill had been voted down, making it a possible double dissolution election trigger.

The Coalition has pointed to interim findings from the Royal Commission into Trade Union Governance and Corruption it established as evidence of the need for the changes.