The Federal Police will halt part of its investigation into the CFMEU ACT branch until a court can rule on the legality of a raid last month.
About 20 police attached to the trade unions royal commission swarmed the construction union's Dickson headquarters in search of evidence of bribery and blackmail on August 26.
Police stayed for about 13 hours and seized about 10,000 electronic and hard copy files.
But the Construction, Forestry, Mining and Electrical Union has launched an ACT Supreme Court lawsuit to have the material returned or destroyed.
Court papers said the CFMEU alleged the AFP raid and confiscation of information was unlawful.
The matter appeared before Acting Justice David Robinson on Thursday afternoon.
Lawyers for the Federal Police voluntarily undertook to not access or examine seized materials – currently held at Woden police station - until after the court could rule on the legality of the seizures and the decision of a Canberra magistrate to issue a second search warrant shortly before 9pm.
The court heard this would require AFP officers to temporarily stop part of their investigations into allegations of illegal activity among Canberra union organisers.
Acting Justice Robinson ordered the matter be heard on October 12.
The judge reserved costs.
Outside court, CFMEU ACT branch secretary Dean Hall said he was pleased the lawsuit had passed the first hurdle and would now go to hearing.
"We believe [the raid] was a massive overreaction by the Australian Federal Police, we've had over 80 notices to produce through the royal commission, and we've complied with every single one," Mr Hall said.
"We don't think the first warrant was executed properly, and the second warrant wasn't even necessary.
"Any information [the police] have gained or have got access to from the search warrant could have been obtained through a notice to produce. We have never not met the notices. We have nothing to hide."
The raids followed criminal charges being laid against two men connected to the CFMEU ACT branch.
Former CFMEU organiser Halafihi "Fihi" Kivalu had pleaded not guilty to two charges of blackmail after he told a royal commission he accepted payments of $60,000 from a Canberra formwork contractor.
Union organiser John Lomax is also fighting an allegation of blackmail in the ACT Magistrates Court.
But Mr Hall disputed the raid had been a legitimate part of that investigation.
"The CFMEUs position is we will not tolerate corruption and will move to eliminate it straight away when we do find it," he said.
"We don't think that this is part of it, this is about a politicising of the Australian Federal Police to try to criminalise the work done that the union does in the area of organising better wages and conditions, and safety on construction sites."
Mr Hall said some of the material seized was sensitive information about the Canberra construction sector that was of no benefit to the police investigation into criminality.