The Federal Court has fined the CFMEU and two officials for trying to force workers to sign up and join the union at a central Queensland building site.
The union has been fined $45,000 for coercion, misrepresentation and contravening workers' rights to shun union membership.
Union organiser Jody Moses told workers on the Gladstone Boardwalk project that it was a union site, and that they'd be kicked out unless they became members.
Mr Moses has been fined $5500, and CFMEU delegate Gregg Churchman $4000 for doing nothing to stop the illegal threats back in 2013.
Judge Michael Jarrett said the incident demonstrated "flagrant disregard of workplace rights and the freedoms of associations guaranteed under the Fair Work Act".
Australian Building and Construction Commission chief Cathy Cato said the court had made it clear that unions would not get away with threatening behaviour.
"Threatening workers to join a union has no place in any Australian workplace," she said in a statement.
"The evidence of workers complaining of being bullied by the union was extremely concerning."