Taxpayers look set to be slugged $20 million to fit out a new office building in regional NSW for the Tax Office, despite the ATO still not being sure about what it will be used for.

A parliamentary committee will examine the spending to put the finishing touches on the controversial office block to be built on the Gosford waterfront, which is set to cost taxpayers a total of $72 million.

Community groups in the central coast town are furious about the site of the project, which is being built on land that was earmarked for a performing arts school.

The committee might be the last chance for those who are opposed to the building, or questioning aspects of the deals done to make it happen, to push for transparency.

There was been no business case or cost-benefit analysis, as required by the Finance Department's rules, for the plan to build a 6500-square-metre office block, nor did the Public Works Committee hold an inquiry into the lease itself.

The committee warned on Wednesday it might hold its deliberations behind closed doors.

The Taxation Office has 6200 desks sitting empty in its buildings around Australia, has shed to more than 4000 jobs since 2013, and is trying to get out of leases on office space equivalent to 2½ times the size of the Melbourne Cricket Ground's playing surface.

The agency, which was hit with a fine last year by the Finance Department for its lacklustre performance in managing its property portfolio, is being forced to relocate up to 500 public servants to the central coast by the end of 2017, when the building is set to open.

The ATO has been unable to say which business units will be moved to Gosford, but Commissioner of Taxation Chris Jordan told the Senate estimates committee on Wednesday there was no intention of moving large numbers of public servants from Canberra or anywhere else.

"There seems to be this myth that we're moving everybody up there from Canberra, or something," he said.

"We are looking to recruit the vast bulk of people from that local area."

In answers to questions from Labor senator Deborah O'Neill​, who has described the process as a "dirty deal done in the dark" , Mr Jordan's colleague, Geoff Leeper, said the ATO had followed standard procurement practice.

"This has been, from our part, a completely standard process. We engaged KPMG as a probity auditor, who gave us a clean bill of health in July of 2015 about how the procurement process had been conducted," Mr Leeper said.

The Tax Office has already made its submission to the public works committee, but the document sheds little light on the question of exactly what work will be done at Gosford.

"The site provides the opportunity for the ATO to lead the redesign of shopfront services on behalf of other government agencies and will continue to foster willing participation with the Australian tax and superannuation systems," the submission reads.

"The Gosford premises will incorporate a co-design centre within a new innovation space, ensuring that members of the regional community have the opportunity to be involved in the co-design process of ATO products and services other co-design centres are located in Canberra, Brisbane and Melbourne."