The Electrical Trades Union wants to know why it is named in the terms of reference for a royal commission into union corruption.
National secretary Allen Hicks has told members in a letter the union knows of no credible allegations against it and had no prior warning it would be named in the document.
Prime Minister Tony Abbott on Monday announced the inquiry, headed by former High Court judge Dyson Heydon, would specifically examine allegations against five unions including the ETU.
Mr Hicks said while the union stood against corruption and would co-operate with the inquiry, any allegations should be dealt with by police.
He said at no stage had the government raised concerns directly with the ETU.
"We have received no formal notice or communication regarding the basis for our naming," he said on Wednesday.
"At no time has the Electrical Trades Union been prosecuted for financial mismanagement or corruption."
Insisting the union had nothing to hide, Mr Hicks warned his members to expect to see "sensationalised reports" coming out of the inquiry.
He said no inquiry by a "hostile and vengeful government" would change the proud history of his union and its job of standing up for workers.
