Government agencies have spent more than $600,000 of taxpayers' money to defend themselves over controversial raids on union offices.
The Registered Organisations Commission has notched up more than $434,634 in legal fees defending itself against the Australian Workers Union during a case over two October raids in Melbourne and Sydney.
The AWU has accused the industrial watchdog of a politically motivated investigation into union donations that led to the raids.
Labor's Doug Cameron grilled ROC commissioner Mark Bielecki in Senate estimates on Wednesday.
"Do you think it's appropriate for taxpayers to spend nearly half a million dollars in lawyers' fees on a witch-hunt into a $100,000 donation made more than a decade ago?" Senator Cameron said.
But Mr Bielecki said the commission had been compelled to defend the allegations, which are strenuously denied, by the AWU's court action.
"What would you have me do, senator? Would you have me not defend the ROC?" Mr Bielecki asked Senator Cameron.
It comes after it was revealed the Fair Work Ombudsman spent about $180,000 to fight a subpoena in the same case despite having 50 in-house lawyers on the books.
"There is no doubt this has put a dint in the funds available for legal costs," Fair Work Ombudsman Natalie James said.
The raids were thrust back into the spotlight on Wednesday, with a subpoena reissued to cabinet minister Michaelia Cash over her office's involvement in the incident.
The ROC's annual budget is $6.5 million, meaning 6.6 per cent of its 2017/18 allocation has flowed to lawyers.
Mr Bielecki said he couldn't estimate the final size of the legal bill.
Senator Cameron asked if the investigation into the AWU was sparked by Senator Cash's staff sending media reports to the ROC.
But Mr Bielecki declined to comment because the matter before the court.
The case is due to return to the Federal Court on August 1.