Activist group the New Zealand Taxpayers Union says it is "willing" to complain to the Ombudsman in the first instance.
"But the council really ought to save everyone the time and expense of an investigation by fronting up with the figure," says NZTU campaigns manager Louis Houlbrooke.
This is ratepayers' money," said Houlbrooke. "When the council makes a screw-up, ratepayers need to be able to see the cost of that mistake in order to effectively hold the Council to account and ensure the mistake isn't repeated."
Public law specialist Associate Professor Dr Dean Knight, of the Faculty of Law and New Zealand Centre for Public Law at Victoria University, said it's not uncommon for local authorities to enter confidential settlement agreements about such building matters.
But he said it's up to the local authority to decide whether doing so is consistent with its idea of appropriate civic and governance culture, especially, and quoting the Local Government Act 2002, the statutory obligation to "conduct its business in an open, transparent, and democratically accountable manner."
"The answer to that might depend a bit on the size and significance of any settlement and associated public interest in what's gone on, especially if the problems were systemic" he said.
The settlement involves a complex of over 100 apartments which was completed in 2007. Legal action was begun by a Body Corporate in 2012.