The construction union is taking federal police to court over a 13-hour raid on its Canberra offices.
Officers seized computers files and hardware in August, following the arrests of a CFMEU organiser and former organiser in the wake of evidence given to the royal commission into trade union corruption.
The CFMEU claims the search was unlawful and is seeking an urgent injunction to stop police from examining or using the material until its case can be heard.
The union's national construction secretary Dave Noonan says the raid was both unnecessary and unlawful.
Staff had complied with a huge volume of commission notices to produce documents.
"If the royal commission police taskforce wanted further material, the royal commission could have issued further notices rather than storming our office," Mr Noonan said in a statement on Wednesday.
Officers obtained a second warrant to continue the search after 9pm on the basis it was to to "prevent the concealment, loss or destruction of property".
"We say that warrant was invalid because there was no evidence to justify that finding," Mr Noonan said.
The CFMEU will seek orders that police return all seized material.
Organiser John Lomax and former organiser Halafihi "Fihi" Kivalu have pleaded not guilty to blackmail charges and are on bail.
The raid and the charging of Mr Lomax were part of a coordinated campaign to use the police against the union's legitimate industrial work, Mr Noonan said.
Federal Opposition Leader Bill Shorten said he wasn't aware of the legal action.
"But I will say that we have zero tolerance in Labor for any wrong doing, any corruption - it doesn't matter if it's an employer, the big end of town or a union representative," he told reporters in Canberra.