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People in the upper South Island may need to get their umbrellas out from Tuesday night to Thursday morning.
Forecaster WeatherWatch.co.nz is warning inland parts of both islands - Bay of Plenty, South Waikato, Taranaki and Nelson - could get a month of rain in just seven days.
Flooding is also possible for both islands, according to the weather service.
The places sheltered from the heavy dumps include Northland, Gisborne, Southland, Otago and southern parts of Canterbury.
However, the rain may not arrive in the region arguably most in need of it, with Auckland - which has had water restrictions in place since May - right on the cusp of the heavier falls.
Auckland could instead be in for heavy wind and even gales from the north and west, which could result in some power cuts from falling trees.