Council documents says appropriate due diligence was done on the building before the 2012 purchase which "identified an issue" and was "part of the negotiation on price".
However, the problem is worse than originally thought.
The council finance and performance committee this week voted through a recommendation to bring forward $4.7 million from the 2015-25 long-term plan budget and put it into safety and enabling works.
Councillor Chris Darby, who was at this week's vote, says he is concerned about what could become "a scary number" and wants lawyers to make sure all liable parties stump up - not just ratepayers.
"I've got questions about how much the total cost is going to be, but at this stage the answer is not clear. I've asked that council's legal team to exhaust all opportunities in seeking warranties for the building at the time it was built and every bit of possible documentation that could hook in another party.
"My hunch here is we're going to be staring at a scary number. We have to be honest about this, we will only really know the scale of what needs to be done when the first slabs start coming off to check the corrosion of fittings," Darby said.
Barriers have been around the base of the council tower since December 21 and last year it was recommended the granite slabs from the eight columns on the tower and around the foyer area be removed.
Councillor Cameron Brewer says a number of elected members want to sell up and cut the city's losses.
"There's an increasing view among councillors that once it's fixed up perhaps it would be better for council to sell and be the main tenant instead. That would free up some serious capital for actual projects of community benefit and would give greater certainty of costs going forward, Brewer said.
"We got it for a good price, we've spent money on it, it has a CV of $133 million and a book value of $150 million. If you could sell it for around $200 million you'd be crazy probably not to."
