The Australian Workers Union (AWU) is targeting another of senator Michaelia Cash's former staff in its bid to stop an investigation into donations by the union, demanding her ex-chief of staff give evidence in the Federal Court.

Key points: The AWU used a private investigator to search for Senator Cash's former staffer Ben Davies in order to serve him with a subpoena

The AWU used a private investigator to search for Senator Cash's former staffer Ben Davies in order to serve him with a subpoena It wants the investigation stopped, after it was revealed Senator Cash's office tipped off the media about the AFP raids of AWU offices in 2017

It wants the investigation stopped, after it was revealed Senator Cash's office tipped off the media about the AFP raids of AWU offices in 2017 Union lawyers have previously filed subpoenas demanding Senator Cash, media advisor David De Garis, and Registered Organisation Commission media adviser Mark Lee give evidence

The union said it hired private investigators to find Mr Davies — something his lawyers said was unnecessary — in an effort to serve him with the documents demanding he give evidence. That subpoena has not been issued.

His lawyers argue Mr Davies does not have any information relevant to the case.

It is over a year since the Australian Federal Police (AFP) raided the Sydney and Melbourne offices of the AWU, during an investigation by union watchdog the Registered Organisation Commission (ROC) into donations the union made to activist group Get Up.

Senator Cash, then jobs minister, and her office came under fire when it was revealed her media adviser at the time, David De Garis, had tipped off television networks that the raids were about to begin, putting them in prime position to record plain-clothed officers entering the union offices.

The AWU launched proceedings in the Federal Court to have the investigation halted and seized documents returned.

Union lawyers have already filed subpoenas demanding Senator Cash, Mr De Garis and ROC media adviser Mark Lee give evidence when the Federal Court hears the case in early 2019, after a number of delays pushed the trial date back from August this year.

On Friday, the AWU added Senator Cash's former chief of staff, Mr Davies, to its list.

The AFP launched an investigation into the media tip-off shortly after the raids, and delivered a brief of evidence to the Commonwealth Director of Public Prosecutions (CDPP) to consider charges.

Senator Cash has consistently and vehemently argued no wrongdoing in the matter, telling both the Senate and Senate Estimates she was not aware of her staff's actions.

The AFP has repeatedly refused to comment on their investigation when questioned by Labor senators, arguing it would be inappropriate to do so given the CDPP is considering the merits of any further action.

Senator Cash also promised to have her lawyers request subpoenas demanding she give evidence to the court be set aside.

The Federal Court is due to hear the case in February.

CORRECTION: MR BEN DAVIES

In November 2018 the ABC published an article about ongoing court action by the Australian Workers Union to shut down an investigation into donations, which was also discussed on the Insiders program. Those publications said that the Union had retained private investigators to "track down" former political staffer, Ben Davies, to subpoena him. The ABC wishes to clarify that it did not intend to suggest that Mr Davies was evading service of the subpoena and acknowledges that Mr Davies' solicitors had approached the Union to accept service. The ABC apologises for any suggestion to the contrary.