Tax Office bosses have pledged for the first time to preserve existing workplace rights and entitlements for their 19,000 public servants as the ATO tries to bring an end to its long workplace war.

The Department of Human Service, said on this week that it was prepared to to preserve "virtually all existing staff entitlements" in a new enterprise agreement for its 36,000 public servants.

The statements are a significant departure from the approach of government agency's since the Abbott-government began to try forcing its tough bargaining policy onto its public servants in 2014.

The main trade union for both agencies, The Community and Public Sector Union, said real progress was being made at Tax but the position at Human Services was more complicated with "deal breakers" still on the table.

The Tax Office and Human Services have both been at the forefront of the three-year industrial struggle between the Coalition government and much of its public service workforce.

Human Services, the government's largest department, has seen allegations of a death threat against a human resources boss while internal bulletin boards at the Tax office were taken off-line at the height of its dispute as workforce rage boiled over. 

But ATO negotiators told public servants last week that they hope to have a new deal on the table and put to a vote before the end of June.

Crucially, human resources executive Lina Ranieri promised workers the conditions and entitlements contained in the 2011 agreement, which remains in force, would be protected.

"If progress continues as planned, we will be offering a draft proposal for you to consider that preserves your rights and conditions that you currently have in the 2011 Enterprise Agreement," Ms Ranieri wrote.

But the ATO's draft proposal must be signed off by the government's workplace enforcer, the hardline Public Service Commissioner John Lloyd and it is understood the Tax office is waiting for the commission's verdict on its draft agreement.

Ms Ranieri's concession comes weeks after the ATO's minority workplace union, the Australian Services Union, said it may give in-principle support to the deal being developed in the current talks.

The union's Jeff Lapidos told Fairfax that the Tax Office's latest message to its workers was "very significant".

"The ASU welcomes the ATO's plan to offer a new enterprise agreement that preserves employees' rights and conditions," Mr Lapidos said.

"We reached agreement in principle to this in March."

The CPSU has told members that the Department of Human Services' position had started to shift, but the union's National Secretary Nadine Flood told Fairfax that significant sticking points remained.

Ms Flood said the department was refusing to move on the issue of rostering, a crucial point in a department with a large number of public servants working non-standard hours.

"DHS has got majority women, many of them working part time, juggling work an family life, on low wages and without a pay rise for three years and the hours-of-work provisions just have not been fixed," the union leader said.

"If [the department] leave an absolute deal-breaker on the table, they're not going to reach agreement."

Ms Flood also indicated the union was very unlikely to endorse any deal that did not include back-pay,  which is banned under the Coalition's policy.

Departmental spokesman Hank Jongen ​agreed that progress was being made.

"We are making good progress towards finalising an agreement that maintains virtually all existing staff entitlements," Mr Jongen said in a statement.