The cost to fix one of Wellington's most important venues is likely to be $16 million more than first thought.
The extent of the work needed to strengthen the St James Theatre also means it will be closed until September 2020.
The theatre on Courtenay Place was initially expected to reopen late next year.
The longer closure of the theatre meant the biennial New Zealand Festival, one of the biggest events on Wellington's cultural calendar, wouldn't be able to use it for its 2020 festival.
It's the second important venue in Wellington to stay closed for longer than first thought - the city's town hall closed in 2013 and wasn't scheduled to open until 2021.
Council documents show $14.9m was put aside for the strengthening work in the council's long term plan.
Councillors were now being asked to approve an extra $8.1m for strengthening work and $8.6m to go towards renovating the building and upgrading its systems, like air conditioning.
The upgrade would future-proof the theatre, city councillor Nicola Young said.
"If we didn't do that in a couple of years' time we would've have to close the theatre again to do that, so we're doing it all in one go."
She said the arts community and the New Zealand Festival was incredibly disappointed the theatre would be closed for longer.
"It's a real problem for us and we are trying to find some solutions because the St James is a really important part of the New Zealand Festival.
"But at the same time we want to make sure we have buildings that are safe for people," Ms Young said.
The increased costs wouldn't result in a rates hike because it was capital expenditure and the money would be borrowed, with the cost spread over 50 years.
That would mean there could be a small increase in rates every year to cover it, but the cost burden would be spread.
There was no direct replacement for the St James, which was a major venue for the New Zealand Festival, executive director Meg Williams said.
However, the festival was already working to figure out ways to work without it.
"There are obviously a range of venues in this city still in action and we're going to have to get creative and think about how we can use those as much as humanly possible.
"We already use those a lot during the festival so that's something we're going to have to work through," she said.
Ms Williams said pop up venues were also an option and that the festival was good at putting on shows that happened outside of traditional spaces.
Discussions for alternatives were already underway.
"The focus was to deliver a fantastic festival for 2020," Ms Williams said.
She supported the work was doing at the St James and said its closure posed a significant challenge for the wider industry.
Usually about a third of ticket sales for the entire festival were sold for performances at the theatre.
Wellington Economic Regional Development Agency's general manager for events and experience, Warrick Dent, said the theatre was important to the city, and once it was renovated and open again, it would be a great asset to the city.
WREDA was working closely with the festival to see how it could mitigate the absence of the theatre. That could include shows being held at Shed 6 or TSB Arena.
So far the closure of St James hadn't impacted the number of shows or performances coming to the city, but the other available venues were working almost at capacity.