The South Island's West Coast ranges will be hit by half a metre of rain over the next three days as an intense weather front hits the area.
Strong winds will also hit parts of the south, while an unusual high hovers over the Chatham Islands.
MetService says the first dose of heavy rain will hit the West Coast and Southern Alps late on Monday, with further heavy falls until Thursday.
More than 500mm in total could fall in the area this week.
The front would disappear sooner, but for an unusually intense anti-cyclone of 1044 hectopascals sitting across the Chatham Islands.
It has been more than 20 years since a similar reading was last recorded in New Zealand.
Winds of up to 140kmh lashed Canterbury, Otago and Southland on Monday and may damage trees and power lines, on what is supposedly the first day of spring.