Dunedin leaders say they can not believe the government's estimates for quake-strengthening the city's heritage courthouse.
Courts minister Amy Adams said the draft business case showed the cost of earthquake strengthening the Category 1 heritage building had jumped to more than $15 million, but she would not release details.
The courthouse was closed in May.
A lawyer, Anne Stevens, said she had complained to the Ombudsman about the refusal to release the Ministry of Justice's draft case, because she said it must be grossly inflated or hiding something.
"I'd be most surprised if the Ombudsman didn't agree with my request because... we're talking about a business plan for a courthouse, we're not talking about a nuclear warhead here."
Dunedin councillor and past Labour government minister David Benson-Pope, said the secrecy was not good or justified.
"We're being fed a lot of porkies here, and I guess people are getting increasingly unhappy with that," Mr Benson-Pope said.
"It's time we had some facts and had the opportunity to contest what we know and the absolutely ridiculous figures we are being quoted."
Mr Benson-Pope said every engineer in Dunedin that the council had consulted had estimated the work could be done for less than $5 million.
There is growing fear in Dunedin the government has already made up its mind to close the courthouse and is using the process as a smokescreen.
Earlier reports from the Ministry of Justice showed the original strengthening estimate in 2011 was $2.5 million, but it has given scant explanation as to why that cost was now six times higher.
The ministry said the costs had simply grown as engineers had discovered the project's true complexity.
It said the courthouse is a Category 1 heritage building with an irregular geometry, and the foundations would have to be underpinned against liquefaction.
The Ombudsman's office said it had Ms Stevens's business case complaint, and was beginning investigations.