The budget for Christchurch's Town Hall restorations has blown out to $152.2 million, after previous assurances it would not increase from $127.5m.

The budget for the delayed Christchurch Town Hall project has blown out to more than $152 million – and the city council tried to cover up the latest increase.

Three years ago the Christchurch City Council told ratepayers they "did not expect" to pay more than the original $127.5m restoration cost.

But councillors met in secret last week to discuss the latest $12m top-up, which takes the total budget to $152.2m, after it already ballooned to $140.6m.

Mayor Lianne Dalziel and senior council staff agreed to answer questions on the blowout after the information was leaked to Stuff. A public-excluded council meeting agenda showed it was proposed to keep the bust private until December 2019, when the project was due for completion and the full cost would be clear.

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Council major facilities capital delivery manager Alistair Pearson confirmed the increase to $152.2m and said extra costs would fall on the council.

"It's been an evolutionary process right through, so as we peel back the layers we find issues, we find concerns, then we then action those concerns and expend money," he said.

"There are other elements where we said 'If we did these functionality improvements now, would that be the right time to do them?'."

The council "added in" new features such as the wind lobby and improved acoustics, the conference room and the upper levels, he said.

The wind lobby was an addition similar to the city council building's Hereford St entrance. The new Christchurch Symphony Orchestra rehearsal facility had also contributed to extra costs, Pearson said.

Dalziel suggested some major spending decisions were made without councillor approval.

"Generally speaking, I think people (councillors) were disappointed it was coming in over what had been budgeted for and how close that information became available to a decision [on the increase] being required," she said.

"There were clearly decisions that had been taken along the way that hadn't been through a proper formal process.

"I'll mention the fly tower … because that was a substantial sum of money … that wasn't formally signed off by council, so I'm also disappointed about that."

GEORGE HEARD/STUFF Christchurch Mayor Lianne Dalziel says the hall budget blowout has "disappointed" councillors, but she does not regret approving restoration.

A fly tower is a backstage system that controls sets and other components of shows.

However, Dalziel did not regret approving the restoration.

"What I'm always reminded of is if we'd gone down the pathway of building a brand new auditorium it would've cost a lot more."

Former Rebuild Minister Gerry Brownlee, who was not convinced the hall was worth restoring, said he was "not surprised at all" to hear about the budget blowout.

"Firstly, the concern we had was just how challenging the land was, because it didn't have the greatest of geotech recommendations on it," he said.

"The Town Hall decision was a nostalgic one; you know, it might work, it might not. I won't make a comment about that, but I'm not surprised it's cost more."

JOSEPH JOHNSON/STUFF Former Rebuild Minister Gerry Brownlee had concerns about restoring the Town Hall and was not surprised by the ballooning budget.

Council customer and community general manager Mary Richardson said it was possible the final cost of the project might be less than $152.2m.

Extra costs would not have an impact on rates and would be found within existing budgets.

Meanwhile, changes have been made to the restoration, including adding two council general managers to the project steering group.

"We are looking closely at the way we run all the projects in the future," Richardson said.

"There were a lot of variations that were made along the way and were a lot of things that we're still waiting for final prices on, so there was potential for the price to go up as decisions were being made.

"Council probably wasn't fully aware of the potential for escalations early enough."

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