Senator Cash faced the Federal Court on Friday to give evidence in an Australian Workers' Union lawsuit against the Registered Organisations Commission, of which she once had oversight.

AWU's case comes after the union's Sydney and Melbourne offices were raided by police in October 2017 as part of an investigation by ROC, the union watchdog.

The union claims the investigation was instigated by Senator Cash and was politically motivated in a bid to hurt Mr Shorten, and was therefore unlawful.

Senator Cash on Friday claimed to have had no interest in the outcome of the investigation and denied she wanted to discredit Mr Shorten or the Labor Party.

Mr Shorten responded on Saturday, questioning whether the senator was telling the truth.

"Listen, whatever she told the court, I'm sure she believes it," he told reporters in Melbourne.

"I just don't know if anyone else does."

The raids on the AWU's offices drew scrutiny because journalists, who had been tipped off by Senator Cash's then media advisor, arrived before Australian Federal Police.

ROC's investigation was in relation to a $100,000 donation the AWU made to activist organisation GetUp in 2006, when Mr Shorten was on its board.

The case continues on Monday with Senator Cash's former chief of staff Ben Davies set to return to the witness box.

Australian Associated Press