The raid, which took place on Tuesday, was prompted by evidence given to the trade unions royal commission, and come as commissioner Dyson Heydon considers whether to withdraw himself on the grounds of apprehended bias. That decision has now been pushed back to Friday.

It is understood about 20 police – including forensic and IT specialists - attached to the Royal Commission raided the Dickson headquarters at 10.30am Tuesday. They stayed for about 13 hours, until almost midnight.

The CFMEU gave officers access to the branch computer system and about 10,000 electronic and hard copy files, and mobile phones were seized.

An email, circulated among senior CFMEU staff on Wednesday morning advising of the raid, said police frisk searched all officers and staff present, removed posters from wall, and went through the office safe and ceiling cavity.

The email, from a union legal officer, said the CFMEU had a lawyer present during most of the raid and the warrant had been issued by the ACT Supreme Court.

"The terms of the warrant required that the executing officers have reasonable grounds for suspecting that there would be evidence at the premises relating to the commission of offences that have been and continue to be the subject of investigation by the Royal Commission," the email said.

"These include the allegations of blackmail against former CFMEU official Halafihi Kivalu and organiser Johnny Lomax.

"We understand the officers are part of the group of police officers attached to the royal commission although we do not have confirmation of that."

The ACT police confirmed on Wednesday that a "significant amount of computer files and hardware was seized" during the raid on the construction union's office.

They said the search warrant "related to people already before the courts," and they would not comment further.

The CFMEU said the raid was "obviously under the direction of the royal commission".

"It smacks of overkill and a waste of police resources at a time when police are stretched dealing with more pressing issues in our community including terror-related activities," the union said in a statement.

"This is nothing other than a political stunt by the royal commission which is desperately trying to defend its credibility and purpose."

The raids come after Fairfax Media reported this week that criminal investigations are taking place in three states into allegations against the most senior levels of the union's leadership on allegations ranging from receiving secret commissions to blackmail.

In Queensland, the CFMEU's former national president and Labor factional boss, Dave Hanna, quit the union as a major criminal investigation examined allegations that he took secret commissions and kickbacks.

In Victoria, police taskforce Heracles has recently taken witness statements from construction industry figures as part of an inquiry into Victorian CFMEU secretary John Setka and his deputy, Shaun Reardon, with a focus on allegations of blackmail over the union's campaign against concrete company Boral.

In NSW, the state union secretary, Brian Parker, is being investigated by police after phone taps were aired at the union royal commission which revealed his close relationship with organised crime figure and allegedly crooked labour hire firm boss, George Alex.

Lomax, a former Canberra Raiders rugby league player and union organiser, was arrested in July and charged with blackmail. He has entered pleas of not guilty.

Former CFMEU organiser Kivalu will also fight allegations of blackmail after he told a royal commission he accepted payments of $60,000 from a Canberra formwork contractor.

Kivalu was taken into custody after admitting under questioning he had accepted $60,000 in payments from formwork contractor Elias Taleb to help him with contacts in the industry.