Justice Mordecai Bromberg on Wednesday found the AWU had a legitimate reason to see the documents from Senator Cash's office to determine if there was political motivation for the October 24 raids in Melbourne and Sydney.
He found the AWU "has a legitimate forensic purpose" in seeing the communications between Senator Cash and the Registered Organisations Commission.
But Senator Cash will not have to hand over all the documents the union sought, with Justice Bromberg limiting access only to relevant ones.
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Josh Bornstein, lawyer for the AWU, said Senator Cash and lawyers for the government agencies had tried to stop the union getting access to the information it needs.
"If the minister, her former adviser and the ROC have nothing to hide in this matter, then why seek to take every avenue possible to block access to documents?" Mr Bornstein asked in a statement issued after Wednesday's court
ruling.
The union will also get documents from the Fair Work Ombudsman and its media adviser Mark Lee.
"I am satisfied that in seeking each of categories one to five, the AWU has a legitimate forensic purpose of demonstrating that the decision to initiate the investigation was motivated by the political purpose alleged or was directed by the minister," Justice Bromberg said in his ruling.
Mr Bornstein said Senator Cash will have to provide documents, including emails, text messages and records of phone conversations, between her, her staff and the ROC to the union.
The raids drew scrutiny in parliament after the media arrived at the AWU's offices before police, thanks to a tip off from her now-former media adviser David de Garis.
Senator Cash needs to hand over her documents by January 5. Mr de Garis and Mr Lee have until January 11, while the FWO has until January 16.
