The heads of the ATO want workers to give up their resistance to Coalition government's industrial relations policies.

The 18,000 public servants at the Tax Office are voting on a massive workplace ballot beginning on Thursday.

They have been offered pay rises of just 1 per cent per year, and been subjected to weeks of intense campaigning by the CPSU for a no vote.

Earlier in the week, public servants at the Defence Department rejected their enterprise agreement for the third time, this time with an increased no vote of 58.4 per cent.

Tax Office human resources leader Fiona Dillon says ATO workers’ conditions and entitlements were safe under the proposed agreement, but there would be no back pay.

“With very few exceptions, the entitlements you have under the current 2011 agreement have been maintained in the proposed EA or otherwise covered by legislation,” Ms Dillon said.

“As an organisation, we will also continue to seek to enhance the staff experience, as we have done in the past, by making improvements and supporting you in ways that go well beyond what is mandated by the EA.

“With regards to our remuneration proposal the bargaining policy allows a maximum pay increase of 6 per cent reasonably spread over three years, with no option for back pay.

“Our offer gives you this maximum 6 per cent pay rise with 3 per cent on commencement, 2 per cent 12 months later and a final 1 per cent another 12 months after that.”

“If we receive a no vote I expect we will recommence bargaining under the same workplace bargaining policy.”

But the unions say voting for the agreement would be a backward step.

Australian Services Union (ASU) official Jeff Lapidos says a no vote would force Commissioner of Taxation Chris Jordan to seek a settlement.

“We ask that you make a short term sacrifice by voting no again so together we can achieve a new ATO Enterprise Agreement that provides maximum support for your job security and your job satisfaction,” Mr Lapidos wrote in a bulletin to ASU members.

“We are confident that Commissioner Jordan will agree to meet our needs because he needs to get a new Enterprise Agreement in place. The process has dragged on far too long for him as well as for us.

“We haven't been able to get the Commissioner to improve his proposal enough because he formed the view that he has improved his proposal enough to get a new Enterprise Agreement across the line.”