The ATO has confirmed there has 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 in downtown Gosford on the state's central coast.

The Taxation Office has 6200 desks sitting empty in its buildings around Australia, is shedding up to 4700 jobs and is trying to get out of leases on office space equivalent to two-and-a-half times the size of the Melbourne Cricket Ground's playing surface.

Finance Minister Mathias Cormann says the government is keeping an election promise by going through with the building but the opposition says spending tens of millions of dollars of taxpayers' money without due process looks like "naked pork-barrelling".

The Taxation Office has confirmed the project will go ahead, calling for expressions of interest from developers to build the building, but ATO bosses still cannot say what it will be used for.

But Taxation confirmed this week that neither the business case nor cost-benefit analysis, required by the Finance Department for large public service property procurements, was not undertaken for the Gosford project.

The agency plans to move 300 of its public servants into the building when it is completed in 2017 but it is unclear who will occupy the rest of the floors.

The ATO had previously said it had been making progress in reducing the amount of excess offices it rents from private landlords and had plans to offload about 50,000 square metres of space, about two-and-a-half times the playing surface of the Melbourne Cricket Ground.

Labor's shadow assistant treasurer Andrew Leigh said on Thursday that it looked as if the Coalition was spending taxpayers' money to make good on promises made before the 2013 election solely to win the federal seat of Robertson.

"Without a business case or any kind of cost-benefit analysis for this building, it risks looking like naked pork-barrelling," Dr Leigh said.

"If the Abbott Government actually consulted with the tax office, the agency would rather keep some of the 4700 staff that are being forced out the door than spend money on a brand new building when it already has thousands of desks sitting idle.

"If Joe Hockey is so keen to make investments in the tax office, he should start by rethinking the retrenchment of 500 auditors, some of whom will doubtless now be employed to help companies minimise their tax bills."

But Senator Cormann said the ATO was observing procurement rules by undertaking the expressions of interest process.

"The Government is delivering on its election commitment to open a new building on the New South Wales Central Coast, with the Australian Taxation Office leading this initiative," Senator Cormann said.

"Submissions to the expression of interest will be evaluated in line with normal processes.

"That is, expressions of interest will be shortlisted and then a more detailed request for final proposals will occur later this year.

"This is normal practice to ensure the best possible value for taxpayers' money."