The union is taking legal action against the Registered Organisations Commission,arguing the federal police raids on the union's Melbourne and Sydney headquarters in October 2017 as part of a commission investigation were politically motivated and therefore unlawful.
The AWU believes it was instigated by Senator Cash in a bid to hurt the union and Opposition Leader Bill Shorten.
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Mr De Garis told the court on Monday that he first heard about the raids at some time between midday and 4.30pm on October 24,2017.
The raids took place that day.
However,he refused to reveal who told him of the raids.
"I respectfully decline to answer on the grounds it may incriminate me,"he told the court.