The union watchdog and the workplace regulator have spent more than $600,000 of taxpayers’ money to engage lawyers in the Australian Workers’ Union raids case – which is still ongoing.

The two bodies – the Registered Organisations Commission and the Fair Work Commission – revealed the spending on Wednesday, the same day Jobs Minister Michaelia Cash was subpoenaed to front the Federal Court.

Under questioning from Labor’s Doug Cameron, Registered Organisations Commission (ROC) boss Mark Bielecki said the watchdog had spent $434,646 to fight the case.

The money had gone to the law firm Ashurst and two barristers, he told a Senate Estimates hearing.

Mr Bielecki confirmed the ROC’s total budget was $6.5 million, meaning the commission had spent about 7 per cent of its funding on legal fees for the one case.

But he said the commission had no choice but to fight the case, which the AWU brought against the ROC over the watchdog’s investigation into donations made by the union to GetUp in 2006.

“The view I take is the ROC is entitled to defend a case based on allegations it denies and wants tried as soon as possible,” he said.

“That leads to expenditure of money to run that defence.”

The agency has faced accusations of political bias from the opposition, claims an angry Mr Bielecki labelled “atrocious” in response to Senator Cameron on Wednesday.

“So your budget is $6.5 million and you’re spending $500,000 on this witch hunt,” Senator Cameron told the hearing.

The union has applied to the Federal Court to have the ROC’s investigation thrown out. Opposition Leader Bill Shorten led the union at the time of the donations in question.

The controversy spilled over into Question Time on Wednesday as Labor turned up the heat on the Prime Minister, asking why Senator Cash remained in her job.

“I have full faith in the minister, Senator Cash,” Mr Turnbull told Parliament.

Mr Turnbull batted away repeated questions about Senator Cash by accusing Labor of attempting to distract from Mr Shorten’s record at the AWU.

“The issue here is about an attempt to stop the Registered Organisations Commission from finding out whether $100,000 of AWU members’ money was lawfully paid or not,” he said.

Earlier, Fair Work Ombudsman (FWO) Natalie James said her agency had spent $180,000 in legal costs to contest a subpoena to produce documents following a request to the court by the AWU.

“There is no doubt this has put a dint in the funds available for legal costs,” Ms James told a Senate Estimates hearing on Wednesday.

“It comes out of the same bucket of money from which we fund and support all our court action.”

Those costs were in addition to the ombudsman’s 50 in-house lawyers, a FWO official told the hearing.

The ombudsman, which is not a party to the case, was successful in narrowing the scope of the court order, but failed to have the subpoena completely set aside.

The AWU, which launched legal action in October, has requested multiple times that Senator Cash and her former media adviser David De Garis be ordered to give evidence. The most recent order was given on Wednesday.

At a fiery press conference at Parliament, Senator Cash said she would ask her lawyers to fight the subpoena and have it set aside, as the court had done in March.

“I have issued instructions for the subpoena to be set aside,” Senator Cash said.

“I do not intend to play the court process out publicly.”

The case will be heard in Melbourne between August 1 and 3. Senator Cash would be required to give evidence in person if the subpoena is not set aside.