ACT Policing confirmed that a 49-year-old Harrison man had been arrested and charged with one count of blackmail in relation to the Canberra hearings of the Royal Commission into Trade Union Governance and Corruption.

He received watch house bail and will face the ACT Magistrates Court to answer the allegation on August 5.

"Enquiries are continuing and it would not be appropriate to make any further comment at this point," police said.

Lomax, now a member of the Construction, Forestry, Mining and Energy Union, is listed as a witness to appear before the commission next week, but it is not known if this will now happen.

Lomax is the third person to be arrested by police attached to the commission.

He is the second person associated with the ACT CFMEU branch to be taken into custody.

Former organiser Halafihi "Fihi" Kivalu was arrested on July 16. He has been released on bail after pleading not guilty to two counts of blackmail.

The CFMEU said Lomax had been accused of forcing an employer to enter into an enterprise bargaining agreement and that as a result the employer suffered financial loss due to paying workers higher wages.

No allegation has been made that Lomax obtained any personal financial benefit, the CFMEU said.

CFMEU national construction secretary Dave Noonan said the arrest of the union official was "absolute disgrace".

Mr Noonan claimed Lomax had been denied the right to have a lawyer present during the time he was at the police station, and had not been provided with written documents setting out allegations against him.

He argued the accused had only tried to negotiate better pay and conditions for workers.

"It is unprecedented and an absolute disgrace that the Abbott government is using the police to intervene in what is purely an industrial matter," he said.

Mr Noonan said that the government was criminalising the work of unions.

"Anyone watching the royal commission this week will have seen a number of union officials being questioned about going on sites over safety issues and their desire to build memberships, the inference being that this is not a legitimate activity," he said.

"With all the important issues that the police are required to deal with, I expect – as would most of the community – their resources are better utilised dealing with ice related crimes, violence against women or indeed, terror related activity.

"The criminal jurisdiction is not the domain of industrial matters, and this is nothing but another vicious political attack on unions and the right to defend and improve pay and conditions for working people."

Lomax played 65 games for the Raiders from 1993 to 1996, and won the club's player of the year award in the 1994 premiership-winning season. However he missed the 1994 grand final due to suspension.

Lomax then moved to the North Queensland Cowboys and finished his NRL career with the Melbourne Storm.

The Australian Federal Police Taskforce attached to the commission urged members of the ACT and NSW community to report matters relating to union governance and corruption.

The commission is supported by other such taskforces in Queensland, NSW and Victoria.

Contact can be made confidentially via email turc-act@afp.gov.au, online at act.crimestoppers.com.au, or by phone on 1800 333 000.