It is understood that important union witnesses have already given private evidence to the commission around the circumstances in which a Construction, Forestry, Mining and Energy Union senior official allegedly threatened to harm Mr Fitzpatrick after he spoke out about the union's support of a corrupt building company owned by underworld identity George Alex.

Several union staff who have worked closely with NSW CFMEU secretary Brian Parker have already been summoned.

The royal commission on union corruption will next week begin examining the CFMEU in Melbourne and, later in the week, in Sydney.

Mr Parker this year denied he or his union had given preferential treatment, or had an inappropriate relationship with, Mr Alex. But building industry insiders have given evidence in suppressed court hearings - which have been obtained by NSW Police and the royal commission - contradicting Mr Parker's claims. The commission is also expected to examine allegations that Mr Parker's Victorian CFMEU counterpart, John Setka, demanded a Melbourne developer employ his brother-in-law and a friend in return for industrial peace.

Fairfax Media has obtained CCTV footage and voice recordings, and an interview with the developer of Victoria's Pentridge prison site, Peter Chiavaroli, which provide insight into backroom dealings involving union officials and underworld figures. Mr Chiavaroli has alleged that in late 2009, Mr Setka demanded the employment as a shop steward of one of Mr Setka's friends, Anton Sucic, as part of a campaign to unionise the site and place it under CFMEU control.

When the demand was allegedly made by the then assistant CFMEU secretary, Mr Setka was accompanied by Mario Amenta, a concrete company owner who is an associate of gangland boss Mick Gatto.

"I told them I had already made arrangements and had interviewed someone who was going to start next Wednesday. Mr Setka said, 'No, that is not going to happen'.

"He [Setka] didn't ask me, he told me that I must employ his compare, Anton," Mr Chiavaroli said. "If I didn't co-operate and employ people, he suggested that he would shut down the place for good."

Tape recordings obtained by Fairfax Media reveal Victorian union officials abusing non-union workers and describing how employers are pressured to "give a kicking" to non-union members.

The Pentridge site was largely non-union until a worker was killed after being struck by a concrete pump in October 2009. The CFMEU then mounted a fierce campaign to unionise the worksite.

CFMEU national secretary Dave Noonan defended the union's conduct on the Pentridge site, noted that Mr Chiavaroli was a ''Liberal Party donor'' and said Mr Setka has engaged in no inappropriate activity and would make ''no apology for his [Setka's] passionate fight for safety on a site where a worker has just been killed''.

In a statement, WorkSafe said there was "insufficient evidence to make a successful prosecution" against Mr Chiavaroli's company in relation to the death.

After Mr Sucic left the site in 2010, Mr Chiavaroli said the CFMEU directed him to employ Mr Setka's brother-in-law, Ivan Dadic. Mr Dadic was also captured on CCTV footage detailing how the union pressures non-union members to join the CFMEU. "He'd be named and shamed and if that didn't work, more of the site would be shut down," Mr Dadic said.

In another recording, union official Gerry Benstead warns Mr Chiavaroli's company against reporting alleged union coercion to the Australian Building Construction Commission, which held concerns about the site but was unable to convince witnesses to give evidence.

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