The Australian Workers' Union has called for federal police to hand over documents it seized during an investigation into media leaks about raids on its offices which won't result in criminal charges.
The Australian Federal Police referred evidence to the Commonwealth Director of Public Prosecutions last year after media outlets were tipped off in October 2017 about raids on the union's Sydney and Melbourne headquarters.
"The AFP can confirm the CDPP has advised they will not be proceeding with a prosecution as there are no reasonable prospects of conviction," the AFP said in a statement.
AWU national secretary Daniel Walton said while he was disappointed no charges would be laid, the move paved the way for information helping the union in Federal Court action its taking over the raids.
"Given someone had confessed to leaking the information about the raids we thought the likely outcome would be a prosecution commencing," Mr Walton told reporters in Sydney.
The raids drew scrutiny because TV crews arrived at the offices before police after a tip-off from then-employment minister Michaelia Cash's media adviser David De Garis, who later quit over the saga.
The AWU is in the midst of Federal Court action challenging the legality of the police and Registered Organisations Commission raids.
"Our investigation is against the ROC and Michaelia Cash being a minister of the crown improperly using her position to commence an investigation into her political enemies," Mr Walton said.
An AFP spokeswoman said they had commenced a review of the documents subject to the public interest immunity claims.
Police and ROC officials were searching for documents relating to an AWU donation to activist group GetUp when Opposition Leader Bill Shorten headed up the union more than a decade ago.
Senator Cash has consistently denied any knowledge of the raids or tipping off journalists, and has stressed she has not been investigated.
"As the minister previously stated, neither the minister nor her office was under investigation," a spokeswoman for the senator told AAP on Tuesday.
Mr Shorten was careful in his response in the development.
"That's a matter for the AFP, I'm respectful of what the police say," he told reporters in Darwin on Tuesday.
"The journalist got a tip-off, we'll never know who tipped them off, but it wasn't us."
Australian Council of Trade Unions secretary Sally McManus said Senator Cash had hidden from questions about the raid.
"It is not acceptable in a democracy to use the power of the state to harass and intimidate working people's representatives. That kind of behaviour belongs in a dictatorship," Ms McManus said.
Australian Associated Press