Michaelia Cash 'refused to give AFP statement'

Deputy Commissioner Close said Senator Cash and former justice minister Michael Keenan had twice refused to provide witness statements to police about media leaks over the union raids, and that both had been asked "at least twice" to explain their involvement in the AWU raids.

"We wanted to have the opportunity to speak to them both," Deputy Commissioner Close said.

She said that, after the federal police referred the leaks to the Commonwealth Department of Public Prosecutions, it had advised on January 11 that there was insufficient evidence to prosecute anyone.

She agreed that the federal police had believed there was "a prima facie case that a conviction could be recorded beyond reasonable doubt", which remained the view of investigators.

"If we had all of the information available to the Commonwealth DPP, yes," she told the Senate estimates hearing.

"However we also work with the Commonwealth DPP, and they are the authority to determine whether a prosecution should proceed or not. So we accept their advice."

It comes as Senator Cash's former chief of staff Ben Davies appeared in the Federal Court on Monday morning to give evidence about the media leaks, which Senator Cash has denied were politically motivated.

Mr Davies, who worked for the senator when she was employment minister, told the court an unsolicited tip-off about union raids had "obvious political implications" for federal Opposition Leader Bill Shorten.

He said the tip-off about search warrants being executed on Melbourne and Sydney offices of the AWU came unsolicited from Mark Lee, the media adviser to the Registered Organisations Commission.

The commission was investigating the union over allegations of document destruction linked to two donations made when Mr Shorten was the AWU national secretary.

The October 2017 raids on two offices proved controversial when media arrived before federal police officers, tipped off by Mr De Garis - Senator Cash's then-media adviser - who received the information from Mr Davies.

Mr Davies told the court on Monday that the information from Mr Lee, who was being hired as Senator Cash's media adviser, had "obvious political implications".

"The implication was self-evident in that it was a rare occasion in which a regulatory authority seeks search warrants in relation to the destruction of evidence," he said.

"There's obvious political implications as well, that the investigation relates to the period during which Mr Shorten was secretary of the AWU."

It was significant that there was an "obvious inference" that the union had covered up or attempted to destroy evidence that was unfavourable to Mr Shorten, Mr Davies added.

The AWU is taking action against the commission, claiming the raids were part of a politically-motivated investigation and therefore unlawful.

The investigation began after two referral letters were sent by Senator Cash, who as employment minister at the time had oversight of the commission.

Mr Davies said receiving the information had been unexpected and, while he conceded it had the ability to compromise the investigation, he didn't feel the need to keep it to himself because "it was in circulation".

He said he passed it on to Mr De Garis but not to Senator Cash because she was in meetings and the source and veracity of Mr Lee's tip were unknown.

Senator Cash told the hearing last week that the first she knew of the raids was seeing them unfold on television.

The cabinet minister denied during a heated questioning that she'd referred decade-old union donations to the commission watchdog as part of a political strategy to discredit or embarrass Mr Shorten.

Instead she said that after reading allegations in an August 2017 media report, she felt it was incumbent upon her to ensure the watchdog was aware of the claims.

With AAP