Federal Jobs Minister Michaelia Cash has been issued a subpoena to give evidence to the Federal Court in August 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 has been ordered to attend the court to give evidence on August 1 and provide documents by June 20.
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.
A Senate committee agreed on Wednesday to invite Senator Cash to answer questions about the subpoena.
Labor frontbencher Doug Cameron told the estimates hearing that Senator Cash was engaging in a cover-up by not appearing.
A spokeswoman for the minister told AAP Senator Cash was not there 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.
Earlier on Wednesday, Senator Cameron clashed with Senator Seselja over Senator Cash's absence from the estimates hearing.
"The minister can run but she can't hide on this stuff. She'll eventually have to deal with it so she may as well front up," Senator Cameron said.
But Assistant Jobs Minister Zed Seselja said he wasn't sure if Senator Cash had other commitments.
"I'm not her diary secretary," Senator Seselja said.