About 100 NSW workers could be made redundant over an enterprise bargaining dispute between the CFMEU and building and construction material company Boral.
Boral says the agreement covering workers at its NSW-based De Martin & Gasparini needs to be changed to allow employees to choose their own union and change the timing of rostered days off.
If that doesn't happen, Boral says the current deal won't comply with the incoming Australian Building and Construction Commission laws, meaning the company will not be able bid for federal government contracts.
"No one wants to see employees made redundant but in this case, this action may be the only way to ensure that DMG's failure to comply with the Code does not prevent the entire Boral Group from being able to tender for Commonwealth-funded building work from September 1," Boral's executive manager Ross Harper said on Monday.
The union said it was seeking legal advice about the company's conduct after workers last week knocked back changes to their agreement. These changes would have stripped away conditions and paved the way for the casualisation of the workforce, the CFMEU said.
DMG rejected this claim and said it wanted to scrap a rule requiring the company to provide details of new employees to the union.
Other changes related to the timing of rostered days off and allowing employees to choose their own union, DMG said.
But the CFMEU said the company was trying to remove employees' right to four-day weekends around public holidays.
It also said the changes would have prevented restrictions on the use of contractors and casual workers.
"You've got a company attempting to rip up that current agreement ... and saying to workers, 'if you don't agree to what we've put to you, we're going to make you redundant'," national secretary Michael O'Connor said.
DMG said it would consult with workers and the CFMEU until July 24.
The current enterprise bargaining agreement is due to expire in March next year, the CFMEU said.