An appeal by the Australian Federal Police over a ruling for it to pay court costs over an illegal raid on the construction union's Canberra offices has been dismissed by the ACT Supreme Court.

The raid made headlines on the eve of appearances from Construction Forestry Mining Energy Union (CFMEU) officials, at the Trade Union Royal Commission in 2015.

The CFMEU sued the police, calling for the material seized to be returned.

Officials said much of it was private information, of no relevance to the royal commission.

The court found police had acted unlawfully and ordered the material be returned, and copies destroyed.

But the AFP resisted an order to pay costs.

The CFMEU's ACT branch secretary Dean Hall said today's ruling by the court meant the police would have to pay out more than $200,000.

He said the entire affair was "a disgraceful waste of police resources".

"The warrant was exercised unlawfully," he said.

"It was designed to maximise negative publicity on the eve of our appearances in the Trade Union Royal Commission."