The Australian Workers' Union has been hit with a defamation suit in the middle of its civil trial against the Registered Organisations Commission (ROC) over raids on the union's offices in October 2017.

Key points: The AWU is seeking to shut down an investigation into its donations to GetUp!

The AWU is seeking to shut down an investigation into its donations to GetUp! The Federal Court heard Senator Cash's former chief of staff Ben Davies told her then-media advisor of AFP raids on the union's offices

The Federal Court heard Senator Cash's former chief of staff Ben Davies told her then-media advisor of AFP raids on the union's offices Mr Davies claims the union defamed him in two statements he said implied he had tried to evade investigations into how the media found out about the raids

Documents lodged with the Federal Court in Sydney show Ben Davies, the former chief of staff to Senator Michaelia Cash, is seeking damages from the AWU, its national secretary Daniel Walton and the news website BuzzFeed over comments the union made in October last year.

Earlier, a Federal Court judge presiding over a separate case in Melbourne heard Mr Davies had advised Senator Cash's then-media advisor of an Australian Federal Police (AFP) raid on offices of the AWU in October 2017.

In the case being heard in Melbourne, the AWU is challenging an investigation by the ROC into $100,000 in donations from the AWU to activist group GetUp! in 2006.

Opposition Leader Bill Shorten was national secretary of the AWU and also a member of the board of GetUp! at that time.

When the AFP raided the AWU's Melbourne and Sydney offices in October 2017 as part of the ROC investigation, it was David De Garis, Senator Cash's senior media advisor at the time, who tipped off the media.

The AFP raided the AWU's offices in Melbourne and Sydney in October 2017. ( AAP: Julian Smith )

Mr Davies' claim alleges he was defamed in two statements issued by the AWU in October 2018 that he said implied he had tried to evade investigations into how the media found out about raids on the union a year earlier.

In the statement of claim, Mr Davies said the union's statement conveyed that he "deliberately evaded service on him of a court subpoena in order to conceal his involvement in the Australian Government's brazen abuse of its powers by using police raids to harass and menace its political opponents".

One of the statements issued by the AWU on October 24, 2018 and quoted in the statement of claim said "a year on from the raids … the AWU's investigators have been unable to find Mr Davies".

Mr Davies is seeking damages, recovery of legal costs and interest.

A spokesperson for the AWU said it had referred the defamation action to its lawyers.

"The AWU will be defending the case," the spokesperson said.

Mr Davies is also a witness in the case between the AWU and the ROC.

David De Garis (left) said he learnt about the police raids before they occurred. ( ABC News: Karen Percy )

Mr De Garis told a civil trial on Tuesday that he had alerted the media to the raids after a conversation with Mr Davies.

Mr De Garis had initially declined to answer questions about the source of his information but Federal Court Justice Mordecai Bromberg ruled he had to answer the questions.

When asked on Tuesday in court, Mr De Garis said he was told by Mr Davies about the raids before 4:00pm on October 24, 2017.

The two men had a short conversation in Mr Davies' office at Parliament House in Canberra, Mr De Garis said.

"I honestly don't recall how … or precisely how long it lasted but he told me the information," he said.

Mr Davies is expected to give evidence later this week.

Small Business Minister Michaelia Cash is due to give evidence on Friday. ( AAP: Mick Tsikas )

Union argues investigation had 'political purpose'

The civil trial brought by the AWU against the ROC has heard Senator Cash sent two letters of referral to the body — the first on August 15, 2017 and another on August 17, 2017.

Court documents show she raised questions about whether the donations to GetUp! were "in accordance with the AWU's rules".

The union contends that the investigation was made "for political purpose", AWU lawyer Herman Borenstein SC said during the opening of the trial on Monday.

"The investigation was instigated because it was known to [ROC executive director Chris Enright] that there was a political interest in it," Mr Borenstein said.

"There was a keen political interest on the part of the Senator.

"It was demonstrated in the beginning by the letters, and it's demonstrated again later in October in making the decision and the media exploitation of the decision after it's made."

The AWU is also arguing the ROC did not have the power to investigate events before the organisation came into being in 2017.

The donations to GetUp! and other donations to entities associated with the ALP, including Bill Shorten's election campaign, were made between 2006 and 2008.

Senator Cash will give evidence on Friday.

Asked about the case in the Senate on Tuesday, Senator Cash said she would "absolutely cooperate with the court".

She said the case was between the AWU and the ROC and as it was before the court, it would be inappropriate for her to comment further.

The hearing continues.