The Canterbury flood was the result of a deep low pressure system, an "atmospheric river", and dry soils which couldn't absorb the deluge. "So we are using the past record to calculate the probability of a flood occurring but because of climate change the probability of that flood occurring might be increasing." "Atmospheric rivers more commonly occur on the West Coast of the South Island, but this one made landfall on the east coast instead." Atmospheric rivers had the greatest impact on the West Coast of the South Island, accounting for much of the heavy rain commonly seen there. "With more moisture in the atmosphere the frequency and magnitude of atmospheric rivers making landfall in New Zealand is expected to increase.

Source: Otago Daily Times June 01, 2021 08:03 UTC