No charges will be laid over the tipoff to the media of the raids on the Australian Workers’ Union headquarters, the federal police have confirmed.

The AFP told the federal court on Monday the Commonwealth Director of Public Prosecutions has concluded there are “no reasonable prospects of a conviction” for the unauthorised leak.

Labor has seized on the revelation, questioning why the resignation of David De Garis, a senior adviser to then-employment minister Michaelia Cash, over the matter was not sufficient to prosecute him for the media tipoff.

A spokeswoman for the AFP told Guardian Australia it now “considers this investigation finalised”, six months after a brief of evidence was sent to the CDPP which the deputy AFP commissioner, Leanne Close, had said indicated the police thought a crime may have been committed.

Labor’s industrial relations spokesman Brendan O’Connor said while he respects the independence of police and prosecutors “it is very disappointing that no charges are to be laid in a matter where senator Cash confirmed to the Senate that a staffer confessed”to being responsible for the leak.

Australian Workers Union national secretary Daniel Walton said the development was “mixed news” because while the union was “obviously disappointed” no charges were laid it could now press for documents which were protected by a public interest immunity claim while the police investigated the leak.

In October 2017 police and officers of the union watchdog the Registered Organisations Commission raided the AWU investigating whether donations made to GetUp and Labor candidates in 2005 were authorised under the union’s rules.

Media were tipped off about the raids, leading to a referral to the AFP to investigate whether an unauthorised disclosure of information about the search warrants had occurred.

On 25 October 2017 Cash told Senate estimates her office was not responsible but was forced to correct her evidence when De Garis came forward following a story in Buzzfeed contradicting the minister.

She said her staffer had “in the course of discussions with journalists, indicated that he had received information … that a raid may take place”. Cash said she was advised the information came from a “media source”.

“At least my staff member has offered his resignation—he has admitted he was wrong,” she said.

The AWU brought a federal court case challenging the legality of the raid but police and the Registered Organisations Commission claimed public interest immunity over documents, including correspondence about the raids.

Walton told reporters in Sydney that while the “AFP was looking specifically at the leaks” the separate AWU federal court case will continue.

The union alleges in its court case that Cash improperly used a law enforcement body in the Registered Organisations Commission to go after her political enemies, he said.

On Monday the AFP advised the federal court the CDPP had decided not to proceed with a prosecution and it will review which documents can be released.

The federal court case is listed for a case management hearing on 29 January and a full hearing starting 11 February.

The AWU has subpoenaed Cash to give evidence in the case, although Cash has said she will apply to have the subpoena set aside and consistently denied any wrongdoing.

“Given the minister is not a party to the proceedings, she is not aware of what occurred in the court,” a spokeswoman for Cash reportedly told the ABC.

“As previously stated, neither the minister nor her office is under investigation.”