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The council undertook the development of the commercial building between 2014 and 2018.
It approved a budget of $12.4million as part of its 2015-25 Long-term Plan and the 2016-17 Annual Plan - but the total cost was $18million.
Chief executive Clare Hadley requested an independent review, which was provided to the risk and assurance committee in late May.
Mrs Hadley said the review's focus was on what lessons could be learnt from the Don St development and how it was handled by the council at both staff and governance levels, rather than seeking to place blame.
The report said that while the lessons were high-level, the absence of attention to those matters resulted in the problems that occurred as the development proceeded.
''Council needs to fundamentally rethink its approach to major projects, especially those involving commercial objectives.
''Once the project was commenced, there appears to have been very poor formal communication to council. However, collectively, elected members have to ask themselves, did they ask the right questions.''
One comment in the report indicated ratepayers were the ones to pay for the development, and would do so for some time.
Lessons identified included the need for better reporting for major projects, that changes must be justified and consistent with (and in this case, not counter to) the council's strategic objective of inner-city revitalisation, and that the council should consider the method of delivery (and operational management) of such an investment.