The Dunedin Council's drainage system simply didn't cope with the very heavy rain that caused catastrophic floods this year, the plant manager says.
Many people were left homeless as houses were flooded in South Dunedin in June, leaving the city with a $30 million bill.
Dunedin City Council water services plant manager Chris Henderson said he believed the city's drainage system would never have coped in those conditions.
"We've put together all the information we can, to inform them [members of the Dunedin City Council] of the total impact of the flooding.
"Our modeling shows there was no way the infrastructure could have coped with that amount of water, " Mr Henderson said.
He was sure planned improvements would slightly reduce the impact of future flooding.
"There's some basic improvements we can make but there's not much that could handle that deluge," he said.
A worst case scenario might involve extensive flooding of low lying areas in large scale rain events, particularly if the rain event coincided with high groundwater conditions.
"There's no simple quick fix," he said.
The report is due to be presented to the Dunedin City Council on 30 November.
