Flooding near the Selwyn Huts in rural Canterbury
Flooding near the Selwyn Huts in rural Canterbury.
Some parts of the Canterbury floods were so extreme it was only expected to happen once every 200 years, scientists says.

During the floods, at the end of May, an Environment Canterbury rain gauge in Mt Somers recorded its largest 48-hour rainfall ever, with 526mm. Another rain gauge about 10km away recorded 310mm in the same period.

National Institute of Water and Atmospheric Research (Niwa) climate scientist Dr Trevor Carey-Smith said the long duration of the rainfall, not the short-term intensity of it, was exceptional.

"Most extreme rainfall only occurred in a relatively thin strip along the Canterbury foothills," Carey-Smith said.

The rainfall recorded at Mt Somers, Geraldine and Snowdon was on average only expected once every 200 years. Those rain gauges span an inland 100km stretch of the South Island.

"These results show how widespread this event was," Carey-Smith said.

Nearer to the coast, the rainfall was less exceptional, Niwa found. The 48-hour rainfall recorded in Ashley, Oxford, Darfield and Methven was expected to occur about once every 50 years.

Meanwhile, the rainfall in Christchurch was expected to occur once every 30 to 40 years while the 250mm of rain recorded over three days in the small seaside town of Akaroa was forecast to happen every 10 years.

Carey-Smith said all of Niwa's calculations assumed the probability of extreme rainfall had not altered, whereas in reality, the likelihood of extreme events was expected to increase due to climate change.

Canterbury's extreme rain event, predominantly across May 29 and 31, led to a regional state of emergency and hundreds of evacuations, including the entire town of Springfield.

Ten state highways and 52 schools were closed at the storm's peak. Farmers fled for their lives, with some describing heart breaking scenes of lost stock and a "wall of water" that caused significant damage to their properties.