The Australian Workers Union (AWU)'s legal appeal against an investigation into donations it made to Get Up has been delayed, as federal police examine how the media was alerted to raids on the union's offices.

The AWU has launched court action to challenge the investigation being conducted by the Registered Organisations Commission, arguing it was done for "improper purposes".

The ROC, which acts as the independent regulator for unions and employer associations, is looking into donations made to the activist group Get Up a decade ago.

Federal Court Justice Anthony North ruled the trial could not start next Monday as planned because of the ongoing investigation by the Australian Federal Police (AFP) into how media organisations were alerted to the raids on AWU offices in Sydney and Melbourne in October.

The court heard the AFP investigation was still three or fours months from completion.

The union sought to delay the trial when it was denied access to three confidential AFP affidavits related to the raids.

"It would be … a disadvantage and unfair if the applicant [AWU] were forced to trial without having access to the documents," Justice North ruled.

Union will attempt to subpoena Cash

The union flagged that when the trial does go ahead it will try to subpoena Jobs Minister Michaelia Cash and David de Garis, a former staffer who alerted the media to the raid.

Mark Lee, who was working with the Fair Work Ombudsman at the time of the raids and was due to join Senator Cash's office, is also expected to be called.

Lawyers for the ROC had argued the March 26 trial date should proceed on the grounds the commission's own investigation was being held up.

The ROC has been unable to use the documents it seized in October while the court action is underway.