A WorkSafe inspector, who initially ruled against stopping the pour, said in a statement he had been intimidated by the presence of four or five Construction, Forestry, Mining and Energy Union officials including CFMEU secretary, Dean Hall, at the Mitchell site.

WorkSafe's Joseph Bartlett wrote in his witness statement to the commission: "[Mr Hall] got up close into my personal space and shouted aggressively at me, yelling words to the effect of 'if you don't f---ing close the site and someone dies, you'll go to jail'."

The pour was then stopped. Mr Bartlett is to appear on Monday afternoon.

The commission was told the union returned the following day, telling the lead contractor, Claxton Constructions, life "would be hell" unless Claw Constructions were sacked from the site.

Claw Constructions had refused to sign a CFMEU enterprise bargaining agreement in 2012, objecting to paying $90,000 a year for unskilled builder's labourers.

Robert Rossi, Claxton's site foreman, told Monday's hearing in Canberra, the concrete pour had been under way on the residential unit development on February 25, 2013 when union officials walked onto the site.

"I observed four to five union officials come on site. I know one of them to be Dean Hall and I would describe one of the other officials as being a large, Maori-looking, bloke," Mr Rossi said.

"As soon as I saw them come on site I called WorkCover even though I thought everything on site was safe."

Mr Rossi wanted WorkCover present because he wasn't sure of his rights.

"I said to the group of union officials words to the effect, 'you can all come on site but as long as you come with me'," he said.

"One of the officials stayed with me... the others took off in all directions. At this time I noticed the large Maori bloke walk over to the formwork and grab one of the handrails and start shaking it back and forth with all his might. I heard him say 'this isn't safe!'. I replied, words to the effect of 'it's not a swing'."

Mr Rossi said WorkCover arrived within the hour.

"I spoke to the WorkCover official although I can't remember what his name was. The WorkCover official looked around and stated words to the effect that: 'the pour can continue'."

Mr Rossi and Claxton principal, Zoran Stojanovic, said CFMEU secretary, Dean Hall, objected.

"I saw Dean Hall walk up to the WorkCover official and go off the Richter scale and start pointing in his face," Mr Rossi said.

"Dean Hall was saying words to the effect: 'if anyone gets hurt, it's your fault, you'll be going to jail'."

Mr Rossi said the discussion "was very heated until the union officials took some time out and walked away to have a private discussion".

He asked the WorkCover official Joseph Bartlett if, in view of what happened, should he try to shut down the pour.

Mr Rossi said: "I could see the WorkCover official was shaken up and he replied: 'yes'."

Mr Rossi then stopped the pour.

He said the union's objection was a technical breach: "They stated that the formwork and handrails were not safe. I believe it was a technical safety breach of spacing between the handrails."

Mr Rossi said the following day, three or four CFMEU officials arrived at the site.

"Zoran and I then had a meeting with the union officials in the site office. I made notes in my diary straight after the meeting. These notes record: 'Had meeting with union in office. They want us to work with them eg. get rid of Claw constructions or we going to make life hell for us. If we don't work in together'."

Mr Rossi said: "I remember replying 'how about we put a tape on and record what you've just said?'"

Mr Stojanovic said a CFMEU staffer told him Claxton shouldn't use Claw, that life would be hell if the company continued to do so and that he would give him "the numbers of better subcontractors than Claw".

Mr Stojanovic denied telling Claw's Troy Armstrong: "I apologise but I've been told by the CFMEU I have to get you off the job and they've found for me [another company] to take over".

He said: "Maybe I told him [Mr Armstrong] they suggested, but I had a contract with Troy or with Claw Construction and I used them before. I use them today. In fact, today they're working. I've got no problems with these people."

On March 4, Andrew Solduka, an engineer agreed to by Worksafe, visited the site to check for safety breaches.

"[He] had a look at everything and said everything was fine. Solduka issued a certificate saying everything was fine," Mr Rossi said.