Halafihi Kimonu Kivalu, 39, was arrested after damning covert recordings were played to the Trade Union Royal Commission in July last year, in which he told formwork subcontractor Elias Taleb to give him money so he could "get some people off your back".

Kivalu, a CFMEU official, pleaded guilty on Thursday to two charges of blackmailing Mr Taleb.

He blackmailed Mr Taleb in relation to work at a residential development at Yarralumla between February 2012 and June 2013, and an apartment complex development in Braddon between September 2013 and January 2014.

Kivalu - a union organiser between 2010 and November 2014 - made it clear to Mr Taleb that if he did not pay him $50,000 to secure the Yarralumla site, a competitor would pay and the work would go to them. Mr Taleb believed he needed to make the payments to keep the CFMEU from disrupting work at his sites.

Court documents suggest Kivalu collected tens of thousands of dollars from Mr Taleb.Kivalu appeared for an arraignment on Thursday in the ACT Supreme Court, represented by barrister James Sabharwal and solicitors Rachel Bird & Co.

Initially, Kivalu had pleaded not guilty and planned to fight the charges at trial. But he signalled late last month that he was changing his plea, and a sentencing date has now been set in May.

Kivalu, who is a former president of the ALP's Dickson-Morning sub-branch in Canberra, was present when the CFMEU shut down Mr Taleb's Yarralumla site in late 2013.

When questioned about the shutdown, Kivalu is said to have replied:

"Don't worry, we'll sort it out."

He called Mr Taleb later and, according to court documents, said:

"Look I sorted it, but you are going to have to give me some money, let's do the deal."

Mr Taleb then paid Kivalu $20,000, according to a police statement of facts.