The ACCC says it will stick to its guns in a fight to address boycotts and bullying on building sites, and has called for union members to speak out about dodginess at the higher levels.

The regulator has been criticised for moving too slowly on evidence that the the CFMEU was involved in years worth of boycotts aimed at keeping Boral concrete products out of the market.

The ACCC launched a case against CFMEU in November last year on allegations that the union would not use concrete manufactured by Boral on at least a dozen commercial construction sites in Melbourne.

But the Royal Commission into Trade Union Governance and Corruption put out a report in late 2014 that slammed the ACCC for being “fundamentally defective” by failing to adequately address the long-standing claims.

Reports say the CFMEU boycott has been happening for years, costing Boral as much as $10 million.

But ACCC chair Rod Sims says that the regulator’s lawsuit is the largest secondary boycott case ever undertaken, and needs to be handled with care.

“I know people said we were a bit slow in tackling the CFMEU case” said ACCC chairman Rod Sims to the Australia Financial Review.

“I don't think we were. We were right out of the blocks but nobody wanted to talk to us.”

Despite the Royal Commission’s rebuke, Mr Sims said those proceedings have greatly assisted the ACCC’s.

Sims praised the forthrightness of Boral chief executive Mike Kane, who took the stand to speak publicly on the issue.

“It was only when Mike Kane came out and went more public on it that it opened people up and got them talking about it,” Sims said.

The ACCC boss said he hopes the Royal Commission’s findings will prompt victims of similar union bullying to speak out as well.

“We're hopeful the CFMEU case will encourage others to come and talk to us about secondary boycott activity,” said Sims.

“We do find when we bring a case like this to court, people often come forward and tell you about other related cases, so bringing one case can often lead to more.”