Commissioner of Taxation Chris Jordan says he has independent legal advice that his agency's covert sharing of employment data with a private polling form was legal.

Mr Jordan said the Australian Services Union, which says it will call in the Privacy Commissioner to investigate the massive data handover, is engaged in "groundless mischief making."

Fairfax revealed on Tuesday that the Tax Office supplied its contractor with the names, email addresses, locations of work and pay grades of each of its 19000 employees without their knowledge or consent.

The contractor ORIMA then used the information to build up a profile of which groups of workers were voting against the unpopular Abbott-era proposals for a new workplace deal at the ATO.

But in a strongly worded statement issued on Tuesday, Mr Jordan, said the Tax office had a legitimate reason for every disclosure is made to ORIMA.

The Commissioner also said the ATO had consulted external lawyers who advised that the "processes undertaken are lawful."

"Should the matter be referred to the Privacy Commissioner, the ATO will openly share this advice and detail of our process," Mr Jordan said.

He took aim at the ASU, saying the union was making it harder for the ATO and its workers to extract themselves from the industrial stalemate that has gripped the revenue agency since 2014.

"Yet again we are seeing groundless mischief-making ahead of the greater good," the Commissioner said.

"It is unfortunate that baseless comments – like the vote process being 'rigged' – continue to cloud the truth.

"Imagine if we excluded employees not actually in the office or without remote access during the voting period from voting!

"The comments made today erode the spirit of good faith bargaining and the trust we have with our employees.

"They are not constructive to reaching a new enterprise agreement for our committed and professional workforce."