The federal opposition has voiced serious concerns about a $255 million upgrade to the immigration department's headquarters in Canberra.
The government used its majority in parliament's public works committee to give the project the tick of approval on Monday, despite objections from Labor.
The plan aims to reduce from 12 to five the number of buildings the department leases in the ACT.
The bulk of the money is being spent on fitting out the new spaces, including a start-of-the-art hub near Canberra airport.
Department official Ben Wright told a Senate estimates hearing the project was expected to save about $236 million over 30 years.
"The financial risk of staying in 12 older buildings versus going into refurbished or new buildings, the overall net benefit ... is to the positive," he said.
But Labor MP Tony Zappia told parliament such savings were purely speculative and based on unconvincing assumptions.
Evidence to justify the proposal and its costs was also vague and failed to address concerns raised by committee members.
"The current buildings that the various departments are located within, in our view, may well have been appropriate for future use," he told MPs.
"We were not convinced that they needed to be relocated."
Mr Zappia said the refurbishment costs appeared to be high compared to other floor space leased by the commonwealth in other buildings.
"The Labor members were not convinced by the arguments that those costs were reasonable under the circumstances and that the Department of Immigration and Border Protection required additional refurbishments that generally cause costs to be higher."
While Labor opposed the project, it did not submit a dissenting report.
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