Accusations of industrial blackmail and illegal conduct have come to light in recent sittings of the royal commission into trade unions.

The trade unions probe has given insights into the forces at the top end of Australian industrial relations and worksite disputes.

The head of building materials firm Boral spoke at a hearing last week, which set off a string of debates about who holds the power in Australian building, and just which party is being held to ransom.

Boral boss Mike Kane gave an account claiming the Construction, Forestry, Mining and Energy Union (CFMEU) had forced contractors not to use any Boral cement products onsite, even after it had been ordered by the courts not to play favourites

Kane dubbed the situation “a clear contempt ... of the legal process”, leaving Boral as a victim of the long running dispute between the CFMEU and builder Grocon.

Boral had been supplying cement to around 40 per cent of large CBD high-rise developments in Melbourne, but Kane claims CFMEU intimidation has slashed this figure to less than 10 per cent,

“This is, to me, a blatant antitrust violation and so I don't understand what the issue is as to why the ACCC cannot quickly move in this matter, but they've not,” Mr Kane said, adding that the dispute had cost Boral more than $10 million.

Fairfax media reports that four long-term Boral customers - Equiset, Drive Projects, Oceania Universal and Meridian Construction Services - have ceased ordering Boral concrete as a result of the union’s campaign in Melbourne.

But spokespeople for some of these firms, as well as other major concrete suppliers, say there is no such racket.

“The market is very competitive,” Equiset said in a statement.

“There is no cartel,” Pronto Concrete general manager Fred Moschini said.

“We're just trying to sell concrete.”

Several key companies refused to respond to questions.

There are severe penalties for taking part in industrial cartels.

Individuals face 10 years in jail and fines up to $340 000 per criminal cartel offence, while corporations can face $10 million in fines.

Whistleblower provisions mean the first party to blow the whistle on an illegal cartel can be granted immunity.

This week, long-term unionist Brian Fitzpatrick is expected to testify to claims he and an unnamed whistleblower were bullied and harassed after raising concerns over branch governance.

Mr Fitzpatrick has said he is keen for the cross-examination, after alleged repeated attempts by the union to silence him and others, including claims he was offered $300,000 to leave the CFMEU “quietly”.