Small business minister Michaelia Cash's former senior media adviser, David De Garis, has refused to answer questions about who told him police planned to raid the Australian Workers' Union offices, on the grounds it may incriminate him.

De Garis made the declaration during the first day of the AWU’s Federal Court challenge over the 2017 raids on its Sydney and Melbourne offices, which the union says were unlawful and had an "improper political purpose".



The controversial raids were executed as part of an investigation by the Turnbull government-established watchdog, the Registered Organisations Commission, into donations made by the union over a decade ago when it was led by current Labor leader Bill Shorten.

Court documents show a magistrate approved the Australian Federal Police’s search warrants at 9:40am on Oct. 24, 2017. When police arrived around 4:30pm, a large media scrum was already gathered outside the union’s offices.



Cash denied five times her office had had any involvement during a now-infamous Senate estimates exchange on Oct. 25, 2017 before announcing later that evening that De Garis was behind the leak to the media and had resigned.

Under questioning from the AWU's barrister Herman Borenstein QC on Monday afternoon, De Garis said he first became aware that search warrants were due to be executed "sometime after midday, before 4pm" on Oct. 24, 2017.

When asked who made him aware of the raids, De Garis told the court he couldn't say.

“I respectfully decline to answer on the grounds it may incriminate me,” he told the court.



The majority of the day was spent debating whether De Garis could be asked about his knowledge surrounding the execution of search warrants or if his evidence could be considered parliamentary privilege.

The ROC's lawyer, Frank Parry QC, said the media leaks were not relevant in determining whether the investigation was politically motivated.



Justice Mordecai Bromberg said there were reasonable grounds for De Garis to refuse to answer and offered the former staffer a protection certificate, that would prevent his evidence being used against him in any future proceedings in an Australian court.

De Garis declined and repeated his intention to not self-incriminate.

Borenstein argued it is unlikely De Garis would be prosecuted, given the commonwealth director of public prosecutions confirmed in January that no charges would be laid for the leaks as there was little chance of a successful prosecution. The AFP subsequently dropped its investigation into the "unauthorised disclosure of government information", which carries a maximum two-year jail term.

"The underlying theme is there was a keen political interest on the part of the senator in the subject matter of the investigation," Borenstein told the court.

De Garis's lawyer Jason MacLaurin said he was "just the media officer" and not involved in decision making in Cash's office.



But Borenstein argued that De Garis was "not low-hanging fruit" but was a "significant person" in the senator's office.



De Garis agreed with Borenstein when asked if he regarded it part of his role as Cash’s senior adviser to use his media contacts to advance the minister’s political agenda. He said the pair would meet on a daily basis to discuss media, political, and Question Time strategy.

Justice Bromberg will rule on Tuesday whether the court will compel De Garis to answer.