The construction union alleges a police sergeant, at one meeting in October, told businessmen that Construction, Forestry, Mining and Energy Union officials were all criminals and urged them to sign enterprise agreements with the MBA.

But both ACT Policing and the MBA ACT said the officer attended to inform industry stakeholders of their rights, and at no time did he advocate for or condemn any group.

The Canberra sittings of the Trade Union Royal Commission earlier this year heard allegations of corruption and cartel behaviour in the ACT building and construction sector.

Three men, including two with union links, were hauled before the courts on a range of charges, including blackmail and perjury as a result.

But, in an embarrassing twist, Canberra prosecutors were forced to drop a blackmail charge against union organiser John Lomax.

An ACT Supreme Court decision last week has again thrown police conduct during the investigation back in the spotlight.

Justice Richard Refshauge on Wednesday ruled part of an AFP raid on the CFMEU ACT's Dickson headquarters in August had been unlawful.

The judge found police had withheld information from the magistrate in order to get a second warrant after the first expired.

"In the light of the strict duty of full disclosure, it seems to me that this failure meant that the warrant was issued on a false basis and is therefore invalid," the decision said.

The finding meant the police were forced to return or destroy information seized under the second warrant.

Justice Refshauge also criticised the police for using the bomb squad as an excuse to impede officials from observing the search.

CCTV footage of the raid showed bomb squad members carrying a ladder, McDonald's, and goggles.

ACT Policing is reportedly considering whether it will appeal the decision.

ACT Policing confirmed an officer attached to the TURC had been invited to attend a number of meetings organised by the ACT Master Builders Association during the year.

A police statement said the aim had been to encourage business owners to be aware of their rights and the legislation relevant to their industry.

"This is one of several forums attended by ACT Policing at the request of the MBA," the statement said.

"At no time during the presentation did the member advocate on behalf of the MBA or comment for or against CFMEU EBAs.

"ACT Policing works closely with local Canberra businesses to maintain strong community relationships and deliver crime prevention advice."

MBA ACT also confirmed the officer, who had been part of the botched raid on the CFMEU offices, along with investigators from federal agency Fair Work Building and Construction, had addressed its members about the building and construction industry.

But the peak body said the officer had not been present during discussions about enterprise bargaining options.

Master Builders ACT executive director Kirk Coningham said: "With an ongoing ACCC investigation, we believed it important that the laws pertaining to market manipulation and cartel behaviours be fully explained and understood by members.

"So, while the AFP was not part of the broader EBA conversation their input was and remains related.

"It was also an opportunity for the AFP to appeal for information and leave contact details."

CFMEU ACT secretary Dean Hall questioned what the ACCC investigation had to do with the AFP.

Mr Hall claimed those present at the meetings reported back to the union that it felt like the police opposed any relationship with the union.

"It made people uncomfortable and they came to the union to report their concerns," he said.

"They said it was almost like the police had clearly taken a side against organised labor and sided with the MBA.

"It's a serious allegation that police officers have taken extraordinarily measure of recommending subcontractors have EBAs with the MBA and not the CFMEU."