Records that have stood for 84 years have melted away - New Zealand has just had its hottest summer on record.
The average temperature was 18.8C - 2.1C above the 1981-2010 average, Niwa says.
The figure beat the historical record previously held by the 1934-35 summer of 18.5C.
Fifty-four locations - including Whangarei, Auckland (Western Springs), Tauranga, Whakatane, Rotorua, Taupo, Hamilton Airport, New Plymouth, Gisborne, Hastings, Paraparaumu, Upper Hutt, Motueka, Greymouth, Akaroa, Dunedin (Musselburgh) and Invercargill - recorded their own hottest summer ever.
Thirty-nine enjoyed or endured their second-hottest summer ever and nine their third-hottest.
Niwa says the 2017-18 season was characterised by mean sea level pressures that were higher than normal to the east and southeast of New Zealand, and lower than normal over and to the west of the country.
This pattern delivered more frequent warm northerly and north-easterly winds than normal, consistent with La Nina conditions.
The summer got off to a hot start with nearly the entire country observing well-above average warmth during December, which was the second warmest December on record.
January 2018 was the hottest month on record for New Zealand with a mean of 20.3C.
February was characterised by the passage of ex-tropical cyclones Fehi and Gita, which brought significant amounts of rainfall to parts of the country.
"Temperatures were well above average (>1.2°C above average) in all main centres during summer. Tauranga, Hamilton, and Dunedin observed their hottest summer on record while the remaining centres experienced near-record warmth," said the Niwa climate summary report.
"Rainfall in Christchurch was well above normal rainfall (>149% of normal) during summer. Tauranga, Hamilton, Wellington, and Dunedin observed above normal rainfall (120% to 149% of normal), while Auckland received near normal (80% to 119% of normal) rainfall.
"Of the six main centres in summer, Auckland was the warmest, Dunedin was the coolest and driest, Wellington was the sunniest, and Hamilton was the wettest and least sunny."
Alexandra had the highest temperature of 38.7C on January 30 Mt Cook Airport and Tekapo had the lowest temperature of 0.2C on December 20.
Upper Takaka had the highest one-day rainfall of 297mm on January 17 Akitio had the strongest wind gust of 165km/h on December 19.
Of the main centres:
Auckland was warmest Hamilton was wettest and least sunny
Wellington was sunniest Dunedin was coolest and driest.