The Carillon bell tower at the National War Memorial has been found to be stronger on top and weaker at the bottom - exactly what engineers have previously warned could lead to its total collapse.
But they now say that's "highly unlikely" to happen.
But the newly discovered weakness makes the country's most famous bell tower yet more earthquake-prone, and is a new blow to chances of it reopening any time soon, after it was closed for safety reasons in February.
That's despite six years of expensive strengthening work - when it was also closed - supervised by the Ministry for Culture and Heritage.
The engineering calculations in a detailed seismic assessment finished in April, but still not released publicly, show the lower tower is only 25-30 percent of New Building Standard (NBS).
Anything under 34 percent NBS is quake-prone, and much more likely to fail in a big quake than a new build in the same spot.
The tower's low NBS rating is due to weaknesses around some door and window openings, called horizontal shear failures. These are in the same zone as weaknesses pointed out to the ministry nine years ago.
"These sections of wall have very little horizontal reinforcing steel, leading to 'unzipping' (separation) of the tower into two halves and significant, rapid degradation of strength," the 2011 report by Dunning Thornton said.
Any strengthening must leave the lower tower stronger than the upper, this report said, so that in a big quake the upper tower could act like a "fuse" and partially collapse "to prevent complete overturning".
However, the ministry, believing from that 2011 report that the lower tower was already strong enough (50-60 percent NBS), only strengthened the upper part, with extra steel braces during work from 2012-15.
The upper part is now 40-50 percent NBS.
That is almost double the strength the new assessment gives the lower part, which comprises perhaps 20 metres of the tower's entire 50m height, including the mezzanine and workshop inside.
Asked if this presents a public safety risk, the ministry told RNZ that the 2020 assessment had concluded that, "based on the geometry of the tower, global overturning of the tower is highly unlikely".
The strengthening work included the corroded steel bell frame and 70 tonnes of bells, which took two years until 2018.
But the frame, like the tower itself, was only partially refurbished.
That frame has now been found to be seriously under Code (15 percent NBS), despite the upgrade.
RNZ has previously reported how no detailed assessment of the bell frame was done until this year, though engineers recommended in 2011 that the ministry do one.
The strengthening of the tower itself went ahead after a "simplified assessment" by the engineer.
The ministry denied that anything had gone wrong with the project.
"The initial work carried out in 2012-2015 was not designed to address all the weaknesses in the Carillon," it said in a statement.
Instead, it focused on what it thought were the weakest parts, in the upper tower.
"It was always known that further seismic assessment would be required to inform all the work required."
Despite knowing this, the ministry spent two years disassembling the 15,000-part bell instrument, replacing all the badly-corroded head bolts and parts of the frame - but not all - then putting it back into a tower that had only had "initial work" done on it.
It has said it was "comfortable" reopening the tower to the public in 2018, even though it had not been fully quake-assessed.
The ministry's chief executive, Bernadette Cavanagh, declined to be interviewed.
She did not respond when asked if she had confidence in how the project had been managed, or if it would be managed this same way now that the ministry has just two years to fully strengthen the war memorial under earthquake-prone building laws.
The Hall of Memories behind the tower, is now 100 percent NBS.
The tower came through the large quakes of 2013 and 2016 seemingly unscathed.
There remains scepticism in parts of the property and construction industry about the accuracy of seismic assessments in general.
The assessment guidelines have tightened since 2017, and applying them to the Carillon shows greater quake demands on it. The "simplified assessment" in 2011 was based on 2006 guidelines.
The goalposts have shifted in other ways for the project, according to the ministry's engineers Dunning Thornton:
- The wrong strength was given to the soil under the tower before work began in 2012; it was inferred from map it was class B, but a detailed geotechnical investigation in 2014 found it was class C - softer.
- The 2011 assessment did not consider the interaction between the tower and the bell frames. In 2020, it's been found the frames "increase the seismic demands" particularly in the upper tower.
- Gruntier computing and analysis in 2020 has given a better idea of how force is distributed in the tower.
- In 2011, greater strength was attributed to the tower's concrete than it gets now.
"This combination of increase in demand and decreased reported capacities has reduced the expected seismic performance of the tower," Dunning Thornton said.
The engineers - nine years ago - had advised "it is not difficult to improve tower's seismic performance to 67 percent of New Building Standard by optimising the current seismic behaviour of the tower".
Even getting to 100 percent NBS would require only a "modest" increase in construction and spending, it said.
This did not prove correct.
In the latest change to seismic rules, engineers are now being told by the Ministry of Business, Innovation and Employment to use guidelines called the "Yellow Chapter" to retrofit concrete buildings.
Dunning Thornton had considered the Yellow Chapter when preparing concept sketches for the Carillon from hereon in, the ministry said.
It must be fully strengthened by May 2022.