Rain pouring down in central Auckland. Photo: RNZ / Cole Eastham-Farrelly
This summer looks set to be wetter and windier than usual.
A change in air flow patterns means there will be mixed weather through to March.
In NIWA's seasonal climate outlook from January through to March, a La Niña watch will also remain in place.
La Niña refers to a cooling of the tropical Pacific Ocean and the subsequent shift in global weather patterns, including a strengthening of moist easterly winds blowing towards New Zealand, and a subsequent increase in cloud development and rain over the longitudes.
It is only the second time in 75 years that a La Niña onset has arrived in summer.
It usually forms in winter, peaks in late spring, and then gradually weakens through summer.
NIWA forecaster Chris Brandolino said every La Niña is different, but it would likely bring moist, rainy conditions to northeastern parts of the North Island and reduced rainfall to the lower and western South Island.
"We've been monitoring the prospect of La Niña for a few months now, and it is heading in that direction.
"So basically La Niña, you have unusually cool waters in the equatorial Pacific in one area, and unusually warm waters other places. And that drives the atmospheric patterns. And that has flow-on effects here in New Zealand."
Brandolino said warmer than average air and sea temperatures were also likely for the North Island, with about equal chances for near average or above average seasonal temperatures for the South Island.
He said there was also an enhanced potential for rain events linked to the tropics and sub-tropics to impact the country from mid-January through to March.
Apart from the west and east of the South Island, which may experience extended dry spells, the rest of the country has a chance of above normal rainfall for the next three months.
The change in air flow patterns will also increase the odds for spells of cooler than usual summertime temperatures.
Photo: Supplied / NIWA
Photo: Supplied / NIWA
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