This summer has been New Zealand's third-hottest on record, NIWA says.
The nationwide average summer temperature was 17.9degC, beaten only by the summers of 2017/18 and 1934/35.
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The Summer Climate Summary, released on Tuesday, shows the main cause was above average sea temperatures around the coast.
"Marine heatwave conditions persisted in the Tasman Sea and coastal areas of Hawke's Bay and Canterbury experienced marine heatwave conditions for part of summer," NIWA says.
In addition to the warm seas, summer air flow patterns pushed up temperatures due to a lack of southerlies throughout the season.
"Most notably, the combination of high pressure and hot air masses originating from Australia led to prolonged hot conditions throughout much of New Zealand to the end of January," NIWA says.
"The dry and sunny weather was influenced by a central Pacific El Niño event."
Several locations across the country observed record or near record high daily maximum and minimum summer temperatures during this time.
The highest summer temperatures were in the Bay of Plenty, Waikato and Coromandel. But the top temperature itself was in Hanmer Forest with 38.4degC.
Rainfall was also below normal for many places, which has pushed Nelson, Tasman and the Buller District into drought.
"Nelson observed a 40-day dry spell which was the fourth-longest dry spell on record there (with records extending back to 1862), while Tauranga and Hamilton had 36 consecutive dry days - their third-longest dry spells on record," NIWA says.