One workplace union has taken action at the Commission, alleging the ATO's program of cuts will save it much more than the $96 million it has claimed during wage talks.

Any vote on the unpopular pay offer on the table will now be delayed until the Commission has heard the case and made a decision.

The Tax Office has offered staff a 4.5 per cent pay rise over three years, which it said would cost $96 million.

Under the controversial bargaining rules set by the federal government, the pay increases offered to public servants must be linked to detailed productivity measures so it is clear how wage rises will be funded.

But like most departments and agencies, the ATO's accountants have had a tough time putting together a package of "productivity offsets" they could get past the Finance Department and the Australian Public Service Commission, who are enforcing the financial side of the government's policy.

In correspondence with its public servants and unions, the ATO has been unable to give a detailed breakdown on how it has calculated its $96 million in savings.

Instead, the Tax Office named general productivity initiatives, such as the 'Reinventing the ATO' reform program.

It also cited its moves to tackle its rampant sick leave problem, changes to flexible working hours of executive level 1 staff, and streamlining the enterprise agreement.

But when asked for a detailed breakdown of calculations, the ATO said it was "unable to identify savings which are directly attributable to each of the four components of new productivity savings".

Australian Services Union official Jeff Lapidos has made it clear he believes the ATO was "plucking numbers out of the air".

The union's legal team will appear before the Commission in Melbourne on Wednesday and Thursday in an effort to convince the industrial tribunal to force the ATO to reveal the numbers behind its publicly stated costings.

"The hearing is about whether the ATO should be ordered to provide the ASU with information that would reveal the extent of the productivity and cost savings it expects to achieve over the next four years from its digital transformation program and from rationalising its accommodation holdings," Mr Lapidos stated in a bulletin to his members.

"The ASU's enterprise agreement negotiators believe the ATO expects to make far greater productivity and cost savings than it has revealed to its employees.

"We expect the provision of this information will enable us to negotiate a better outcome than currently proposed by the ATO."

A Tax Office spokeswoman did not answer a question on whether the union official's assertion was true, but said the office had made attempts to clarify its position with the ASU.

"The ATO has attempted on a number of occasions to clarify the various issues with the ASU and while progress was made, some differences on interpretation on several matters remain so there will now be a formal hearing at the Fair Work Commission on October 14 and 15, 2015," she said.