Temperatures are most likely to be above average around the country from October to December, according to NIWA.
In its latest seasonal climate outlook, it said: “Warm air masses will flow from Australia into New Zealand at times during October.
“An increased prevalence of northeasterly winds may lead to more days above 25C in November-December.”
NIWA’s report said La Niña-like patterns favouring more high pressure south of New Zealand over the three-month period would mean dry spells develop, especially in the west of both islands, and bring a gradual reduction in the typical westerly winds.
La Niña is part of a climate phenomenon called the El Niño Southern Oscillation (ENSO) system.
A La Niña watch remains in effect, with a 60-70% chance it officially develops by December.
NIWA said while there would inevitably be a mix of relatively wet and dry days, and hot and cold days within a season, temperatures were “most likely to be above average across the country”.
However, there could be the chance for heavy rainfall events to occur in October during La Niña-like conditions, as low pressure systems become stalled near the country.
It also said overnight temperatures may be “unusually warm” in the east of both islands, as easterly quarter winds become more common than normal during this period.
Meanwhile, rainfall totals were expected to be “near normal” for most regions.
Consistent with La Niña-like conditions, some regions could experience some slow-moving and moisture-laden low pressure systems that could occasionally approach from the north, bringing a chance for heavy rain.
NIWA also said a rise in coastal sea temperatures toward the end of September suggest marine heatwave conditions could also intensify in the months ahead.