The Australian Services Union will make an urgent application to the Fair Work Commission unless the tax office reconsiders its plan to hold a vote on a revised enterprise agreement in late April.

The union believes the tax office have deliberately scheduled a vote during the school and public holiday period when much of the 20,000 strong workforce takes leave.

In a letter to Commissioner Chris Jordan, ASU tax branch secretary Jeff Lapidos said the proposal was unfair and would undermine collective bargaining.

"It would do so because large numbers of ATO employees have families and take leave during school and public holidays," he said.

"Access and voting periods during April would deny those employees the opportunity of participating in the enterprise bargaining process."

But a tax office spokeswoman denied the claims and accused the union of deliberately delaying negotiations.

"We are taking the school holiday periods into account in determining the timing of a vote process," she said.

"We will of course be ensuring that our people have the maximum opportunity to participate in any vote process and this includes facilitating options for those who may be on leave at the time."

Rank-and-file staff rejected an annual pay increase of 2 per cent in December just hours before 200 bosses secured a 3 per cent pay rise.

In an email to union members, Mr Lapidos said the vote on a revised deal was welcome although the timing was problematic.

"If the ATO proceeds with its stated intention then employees taking leave to spend school or public holidays with their family will be disenfranchised in a least one state," he said.

The tax office spokeswoman said the department had made significant improvements to the agreement and management was eager to present it to staff.

"We remain committed to productive dialogue about our enterprise agreement," she said. "We have demonstrated that we have listened to the views of our people and acted on their feedback."

Mr Jordan recently announced plans to drop the demand for an extra 45 minutes work each week and make other key concessions, aimed at breaking the bargaining impasse that has gripped the agency for more than 18 months.

"The new proposal reinstates the conditions that mattered most to you and keeps us at the forefront of APS pay and conditions," he said in an email to staff earlier this month.

"This is a significant shift from the last offer and I hope it shows we have listened to you."

The ASU has also made preliminary steps to prepare for protected industrial action.

Our plan is to encourage members to change the way they work so it results in lower productivity for the Commissioner," Mr Lapidos said in an email to members.

"Our industrial action will be effective if we can reduce the ATO's productivity by at least the 2 per cent per annum we are losing through the Commissioner's game playing that is designed to remove our rights at work."