Building products group Boral is suing the CFMEU for $20.7m over a black ban the union ran in 2013 and 2014.
AAP reports that the company is seeking damages for losses from 16 projects as well as loss in market share and reduced profits caused by discounts they had to make to try to win business.
On the first day of a four-week hearing in Victoria's Supreme Court on Monday, the court heard the black ban arose from the fact that Boral had continued to supply concrete to Grocon, a construction company the union was in conflict with.
Boral's lawyer Stuart Wood QC told the court customers had told Boral staff they would prefer to buy their concrete from another supplier than put up with delays the union had threatened.
Boral is seeking damages for losses up to 2018.
Representing the CFMEU, Rachel Doyle SC said this was "an attempt to perfect the market in its favour".
"The position Boral was in before 2013 was at the mercy of natural forces, it had to compete in a difficult market," Ms Doyle said.
"It's a competitive market where price is king and opposition ruthlessly target each others' customers."
The hearing will return to the Victorian Supreme Court today.