Hawke's Bay and Gisborne could be in for a wetter-than-usual winter, with "particularly unsettled" periods of heavy rainfall forecasted in the next two weeks.
It comes after Cyclone Gabrielle badly hit northern and eastern parts of the North Island earlier this year.
Across the country, NIWA said this year's winter is unlikely to be colder than average, but will probably be colder than record-warm winters in the past few years.
However, the forecaster said there would still continue to be patches of heavy rainfall, snow, and strong winds due to warmer-than-average sea temperatures in the western Pacific.
"This is expected to bring heavy rainfall, snow, and strong winds through June.
"Higher than normal air pressure is forecast to develop in the Tasman Sea during July and/or August, leading to periods of below normal rainfall, particularly in the north and east of both islands," NIWA said.
In Hawke's Bay, Gisborne, and Wairarapa, the agency is forecasting "particularly unsettled" periods over the next two weeks. The regions were hard-hit by Gabrielle.
Over winter, NIWA said there's an about equal chance of near-normal or above-normal rainfall totals for the region over winter. The agency uses the term "about equally" to describe forecasts where probabilities are within 5% of one another.
"Rainfall totals are about equally likely to be near normal (40% chance) or above normal (35% chance). Early-to-mid June looks particularly unsettled, with periods of heavy rain possible," the forecaster said.
"July and August may feature more southwesterly-quarter winds, leading to spells of drier than normal weather."
Meanwhile, NIWA's forecast for Northland, Auckland, Waikato and Bay of Plenty had an about equal chance that there would be either average or below-average rainfall totals.
"Rainfall totals are about equally likely to be near normal (40% chance) or below normal (35% chance)."
Higher than normal air pressure would likely mean spells of drier than normal weather in July and August for the region, the forecaster said.
NIWA added that its forecast confidence for rainfall was low to medium, while its confidence in temperature forecasts was medium-to-high.
"A colder than average winter is unlikely. This is because marine heatwave conditions persist in coastal waters, particularly near the South Island. Winter 2023 may, however, be colder than recent winters as the effect of El Niño builds," the forecaster said.
"While a transition to El Niño is occurring, the persistence of warmer than average ocean water in the western Pacific, a La Niña-like tendency, is supporting and will continue to support an increased frequency of low-pressure systems around New Zealand.
"Some of these disturbances will come with sub-tropical and/or tropical moisture. El Niño-like patterns may develop during July and/or August, allowing for periods of time with less rain, particularly in the north and east of both islands."
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