A several-metre section of the three-tiered wall, built by the Oamaru Beautifying Society more than 80 years ago, failed on the afternoon of May 30 and stone blocks tumbled on to the busy stretch of State Highway 1.
The wall was built as a retaining wall to the standards of the day and, over time, trees and bushes were planted around it for beautification purposes.
It was anticipated that root mats would create additional strength.
At the time of the collapse, Mr Kircher said an inspection identified its cause as weather-related and not a structural defect.
Only two months earlier, the council spent $45,000 on stabilising the wall, including removing leaning sections, reestablishing drainage behind the wall and building replacement sections.
A report to members of the Waitaki District Council's assets committee yesterday said there was no one factor that contributed to the wall's collapse.
Rather, it was a combination of factors, such as no extensive geological assessment, differing soil conditions, backfilling methods, the effect of old root masses and heavy rain leading up to the collapse.
Rainfall resulted in the soil and backfill material becoming saturated.
However, that backfill material had an ''unfortunate weakness'' in that it rapidly absorbed moisture at a higher level, to the point of ''mechanical failure that is well in advance of other soils in the area''.
That added significant weight to the structure and led to its collapse.
An inspection by council staff found no construction or design weakness.
Mr Kircher described the collapse as an ''unfortunate situation''.
''There's some things that went wrong here and we've actually learnt from that. It's a situation if the wall was built to modern standards it probably would have handled what was behind it. We just want to make sure from our point of view we're giving this the right scrutiny and this report pairs with that. This is a case of learning from our mistakes.''
Council staff accepted that more focus should have been placed on engineering design work instead of a ''like for like'' rebuild.
Mr Kircher believed a thorough investigation and subsequent report were required, as serious questions had been raised by the public.
-By Daniel Birchfield
