The report, by property officers and authorised by senior finance executives, assesses the risk profile as "moderate", reflecting the relatively low likelihood that a stone will fall from the building.
However, it says there is the "possibility of life-threatening consequences should a stone fall".
Problems with the stone cladding were flagged during due diligence of the building in 2012, but during refurbishment of the building in 2013, quality issues identified included:
• Non-standard fixing design
• Insufficient, missing or loose bolts/pins
?• Inadequate or missing packing, meaning the pins were supporting the weight of the stones
• Corrosion in bolts and support rails
• Some stone with no mechanical fixing and stuck using epoxy with an uncertain lifecycle.
Additional investigations in 2014 found the fixing on some columns was reasonably sound, but not others. Last year, it was recommended the granite slabs from the eight columns on the tower and around the foyer area be removed.
The report said $4.2 million was tagged when the building was bought to address known issues with the building facade.
A detailed design, due for completion in a month, is being undertaken to address the substantive works.
Options to fix the problem include further investigations, repairing the cladding or replacing it with a lightweight alternative.
Ratepayers have already paid $128.5 million to buy and fit out the former ASB Tower, described as robust, structurally sound with good bones when it was bought in 2012.
Councillor Chris Darby has estimated it will cost tens of millions to repair the building, which opened in 1991.
When the Herald reported on the problems last month, Home Owners & Buyers Association president John Gray said the cost of repairs would be many, many millions of dollars and "could blow out to an extraordinary amount".
New health and safety regulations requiring the installation of full protection for scaffolding was very expensive and would cost millions of dollars alone, he said.
Mr Gray said the experience of large apartment buildings showed once the cladding came off other deficiencies were exposed. He said the repair bill on the Landings Apartment building in Parnell, not as high as the council tower, started with an initial estimate of $8 million for facade and cladding issues, went to $12 million and was now close to $15 million.
The ASB Tower was bought in July 2012 from Brookfield Multiplex for $104 million. A further $24.5 million was spent fitting it out and bringing it up to a five-star green rating.
Councillors were told in August 2012 that the due diligence process had identified the facade and roof as having unexpected areas of premature degradation and a potential repair cost of $4.2 million, but was not a major issue. Senior officers said the purchase would save ratepayers $27 million over 10 years compared with the mix of rented and owned CBD accommodation.
"This is a sound investment we are making on behalf of the people of Auckland", producing "significant savings for ratepayers now and in the future", former chief executive Doug McKay said in July 2012.
