Christchurch City Council has announced fast-track mitigation work following a one-in-100-year storm that caused severe flooding in the city.

Heavy rain last Tuesday and Wednesday flooded homes and businesses, cut power and caused transport chaos.

The Insurance Council of New Zealand (ICNZ) says the incident was “significant”, but it is too early to supply figures.

“It was officially categorised as a one-in-100-year return-event flood,” ICNZ Insurance Manager John Lucas told insuranceNEWS.com.au. “It was the largest since 1975, according to Christchurch City Council.”

The Canterbury earthquakes affected flood mapping for much of the Christchurch area.

“Some of the flat land in Canterbury around eastern Christchurch has dropped by up to one metre, and this has affected the natural and man-made drainage performance,” Mr Lucas said.

He says ICNZ supports the proposed mitigation work.

Christchurch Mayor Lianne Dalziel says addressing increased flooding is a priority, and much work has been done in the 18 months since quake aftershocks eased.

“But Christchurch people have been through yet another extreme event… and it has been a real wake-up call for all of us.”

ICNZ CEO Tim Grafton says the announcement is “good news for hard-pressed residents”.

“It’s also good news for them if mitigation significantly reduces the risk and frequency of flooding, because it supports the future insurability of property,” he said.

“We are also pleased to hear the city council is firming up timeframes for how it will deal with other flood risks, Port Hills slope stability and other hazards long term.”

Mr Lucas said it would probably take another four to six weeks before ICNZ had an initial indication on the total expected value of claims.