Federal Jobs Minister Michaelia Cash claims she is being bullied by Labor, after deciding to challenge a court order to give evidence over raids on Australian Workers Union offices.
The court case relates to two raids in October on the AWU's headquarters in Melbourne and Sydney.
Senator Cash went on the counter-attack on Wednesday after being issued a subpoena to give evidence in the Federal Court on August 1 and provide documents by June 20.
"Today is just another effort by the union movement to protect Bill Shorten," she told reporters in Canberra.
Senator Cash will apply to have the subpoena, which she says is a union stunt, set aside before a June 8 case management hearing.
The raids stemmed from a Registered Organisations Commission investigation into a $100,000 donation from the AWU to activist group GetUp.
Senator Cash said Mr Shorten had questions to answer over the donation, given he was secretary of the union and a director of GetUp at the time.
The raids drew scrutiny after the media arrived at the offices before police, thanks to a tip-off from Senator Cash's media adviser, David De Garis, who later quit over the incident.
Mr De Garis has also been issued a subpoena, along with Fair Work Ombudsman employee Mark Lee and Registered Organisations Commission official Chris Enright.
AWU national secretary Daniel Walton said the union had a long-held belief the raids and investigation were unlawful.
"If we are to understand exactly what happened then we believe the testimony of these individuals is critical," Mr Walton said in a statement.
Senator Cash rejected calls to reappear before Senate estimates after a committee invited her to answer questions about the subpoena.
"I will not be bullied by the Australian Labor Party," the minister said.
Labor frontbencher Doug Cameron told the estimates hearing that Senator Cash was engaging in a cover-up by not appearing.
Senator Cash said she was not at the hearing because she was not responsible for the portfolio, which is held by Workplace Minister Craig Laundy.
Assistant Jobs Minister Zed Seselja, who is representing Mr Laundy in the hearing, said he wasn't aware of the subpoena.
He said he would take on notice if Prime Minister Malcolm Turnbull knew about the court order.
In December, Senator Cash was ordered to hand over emails, text messages and documents linked to raids.
The Federal Court found the AWU had a legitimate reason to see documents from Senator Cash's office to determine if there was political motivation for the raids.
