The Australian federal police have referred material to commonwealth prosecutors relating to a media tipoff about a police raid on the Australian Workers Union.

The employment minister, Michaelia Cash, repeatedly denied that her office was involved but on 25 October her then-senior media adviser, David De Garis, resigned after Buzzfeed revealed he had tipped off media about the raid. The AFP then set up an investigation into the leak.

The referral will further delay the union’s case challenging the legality of the raid while the commonwealth Director of Public Prosecutions decides whether the evidence supports laying charges over the leak informing media about the raid before it took place.

In October journalists including television cameras were in attendance when the police raided the AWU headquarters as part of the registered organisations commission’s investigation into $100,000 of donations from the union to the campaign group GetUp in 2005.

At a case management hearing on Monday, the AFP confirmed it was liaising with another agency about the matter. Guardian Australia understands the police have told the AWU they are planning to send material including witness statements, emails, SMSes and phone records to the CDPP in the next two weeks.

The national secretary of the Australian Workers Union, Daniel Walton, said: “It is troubling, but not surprising, that the Australian federal police have decided to refer their investigation to the commonwealth prosecutor.”

The ROC set up the investigation after Cash wrote to the union regulator on 15 August enclosing a media report about the donation and questioning “whether or not this donation was validly authorised in accordance with the AWU’s rules”.

Walton said “Cash and her government are obsessed with harming unions and their members”.

“Clearly the AFP have learned enough here to believe it warrants the attention of the prosecutor.”

The secretary of the Australian Council of Trade Unions, Sally McManus, said it was a “serious matter to refer something to the DPP”.

McManus said Cash should have resigned when her office was found to have tipped off the media.

In federal court on Monday the police sought to maintain the public interest immunity claim against releasing the material to the union for another six weeks.

In December Justice Anthony North upheld the AFP’s public interest immunity claim, finding that although the documents relating to the raid may help the AWU establish whether the investigation was initiated for an “improper purpose”, releasing the documents may prejudice the AFP’s investigation.

In February, police revealed they had taken witness statements from 14 people as part of the investigation and spoken to thirty-four other witnesses.

Under federal law it is an offence punishable by up to two years in prison to publish information obtained by virtue of a person’s position as a commonwealth officer without authorisation.

Labor senators have pursued the AWU raid matter in Senate estimates but have not established the identity of the “media source” who Cash said had told De Garis about the raids.

Labor questioning has established that the former director of media at the Fair Work ombudsman, Mark Lee, had been told by the ROC at 12.30pm on the day of the raids that warrants had been sought so that he could help field media inquiries.

In October the then fair work ombudsman, Natalie James, said Lee had told her he had “no communications with anyone about these matters outside the agency until after the searches commenced”.

Cash may yet have to give evidence in the AWU’s civil case challenging the legality of the raid because despite vowing to have a subpoena set aside, she has not yet filed an application to do so.

An AFP spokesman said the force “has commenced an investigation into the alleged unauthorised disclosure of information concerning recent search warrants executed in support of a registered organisations commission investigation”.

The spokesman said the AFP “will be making no further comment” about the matters before the federal court and the matter under investigation.

Guardian Australia contacted Cash for comment.

A further federal court hearing is set for 21 August.