The top of the South Island and parts of the West Coast will be in for heavy rain, beginning in the evening. There is a rain warning issued for Nelson, Westland and Buller.
Meanwhile, United States weather analysts have predicted New Zealand's summer in general will be wetter than average, thanks to the arrival of a weak La Nina.
The US National Oceanic and Atmospheric Administration announced on Friday that the La Nina system was expected to last until February.
As the flip side of El Nino, it occurs periodically as central Pacific Ocean waters cool and affect global weather patterns.
WeatherWatch said that despite its cooling effect elsewhere in the Pacific, La Nina would instead slightly warm ocean temperatures near New Zealand.
This could bring cloudier weather in coastal areas, a high chance of afternoon showers, especially in inland regions, and a wetter-than-average summer for the North Island and some western and northern parts of the South Island.
La Nina-powered easterly winds could also drive spillover rain from the West Coast into drought-affected Canterbury, WeatherWatch said, but the effect was likely to be mild because of the weak nature of the La Nina.
Today's forecast:
Whangarei: Cloud increasing with patchy rain developing at night. A high of 22C.
Auckland: Cloud increasing with patchy rain developing at night. High of 21C.
Tauranga: Mostly fine in the morning with cloud developing later on and patchy drizzle. High of 19C.
Wellington: Mostly cloudy with rain developing and severe gales to pick up in the early evening. High of 18C.
