The country is in for a scorching January as a marine heatwave makes its return.
While it is forecasted to be a mixed bag for the main holiday hotspots across the country for Christmas Eve through to Boxing day, the weather is expected to get progressively hotter and more humid, especially after Sunday.
"This will likely have a profound effect on coastal sea temperatures, which are already in a marine heatwave," NIWA said.
Ocean temperatures are currently already up to 3C above average in some areas.
It comes after New Zealand's coastal waters were free of marine heatwaves for the first time in two years, according to Moana Project scientists.
Despite coastal water temperatures being more unusually warm in all regions during the start of November, sea surface temperatures suddenly dropped from November 25, likely caused by the cold, windy weather.
However, with summer about to be in full force, the country's temperatures, both in and out of the water, are expected to climb.
NIWA's summer climate outlook said there is potential for a warming trend during the second half of December which could enable the marine heatwave to strengthen.
The outlook predicted summer temperatures to be near average or above average in eastern parts of the country, very likely to be above average in the north of the North Island and the west of the country, and most likely to be above average in the north of the South Island.
Sub-tropical northeast winds may result in more frequent hot days, over 25C, for inland and western parts of the country, the outlook said.
Warmer than average regional seas are expected to fuel occasional heavy rainfall events, NIWA found. However, during periods of high pressure, dry spells will occur, particularly in inland and western parts of both islands.
NIWA's outlook said there is potential for a warming trend during the second half of December which could enable the marine heatwave to strengthen.
"The NIWA sea surface temperature update suggests that [marine heat wave] conditions are likely to last through summer and possibly into autumn," the outlook said.
"Given the high-impact nature of recent summer marine heatwaves, the marine sector should monitor this situation closely, particularly as it has a higher starting point than 2021 and 2017, two of New Zealand’s most severe MHWs."
