A union cost a concrete supplier $8 million when it banned its products from Melbourne building sites during a “war” with a developer, a royal commission has been told.

Concrete supplier Boral says its trucks were banned from union work sites in April 2013, with the ban continuing to this day.

Boral executive general manager for construction supplies in Victoria Paul Dalton’s witness statement to the Royal Commission into Trade Union Governance and Corruption said CFMEU secretary John Setka banned Boral to put pressure on builder Grocon.

Mr Dalton said he met with Mr Setka to talk about the ban.

“We’re at war with Grocon, and in a war you cut the supply lines,” Mr Setka allegedly told Mr Dalton.

“All wars end and once peace is established the CFMEU will be at the table to divide up the spoils. The CFMEU will decide who gets what and what market share Boral will get.”

Mr Dalton said Boral refused to stop supplying Grocon, and the union ban had since cost the company more than $8 million in lost revenue and legal costs.

“A number of customers would ring up and say ‘we’re not allowed to use Boral anymore’,” Mr Dalton told the commission in Melbourne on Wednesday.

Mr Dalton said even an injunction against the ban from the Supreme Court did not help.

“We may have written protection from the courts, but the final power still belongs to the union,” one customer said in an email to Boral.

Mr Dalton said Boral’s market share was diminished, as customers did not even ask for a quote on a union-controlled job.