Niwa's latest big climate summary, issued today, showed the nationwide average temperature in May was 11.2C - or 0.4C above the 30-year average.
Thanks to more westerly winds than average over New Zealand, which brought a Foehn effect, temperatures in parts of Hawke's Bay came in at 1.2C above average, or "well above".
Hastings, Waipawa and Wairoa all saw their third-warmest May on record.
For everywhere else in the North Island, temperatures were above average, except for Northland, Taranaki and Wellington, where the end-of-month figures were instead "near" average.
In the South Island, May temperatures were above average in Nelson and coastal Canterbury north of Ashburton - Akaroa recorded also its third-warmest May - while temperatures elsewhere in the island were near average, and below average in parts of Southland.
Across the country, the first half of last month was relatively warm and dry.
Twenty-two days in, Palmerston North, Turangi, Waiouru, Te Puke, Motu, Te Kuiti, Waipawa, Wairoa, Farewell Spit, Westport, Culverden, Waiau, Cheviot, Medbury and Akaroa were all on track to break records.
But May took a chilly turn when a blocking anticyclone became established over the southeast of Australia, delivered a prolonged period of disturbed west and southwesterly winds over New Zealand.
Many fronts were embedded in this flow, which resulted in an unsettled spell of weather with bands of thunderstorms passing over the country.
Niwa meteorologist Ben Noll said a series of fronts and cold air "straight from Antarctica" ultimately gave May its chilly close.
The cool change was all the more dramatic given the balmy conditions many Kiwis had been enjoying in the lead-up.
But Noll added some of those pleasant near-to-above average temperatures had returned to much of the North Island over the opening days of June.
That was a flavour northern parts of the country, including Northland, Auckland, Waikato and the Bay of Plenty, could expect for the rest of winter.
Elsewhere, Niwa has forecast temperatures between now and the end of August would likely be above average or average.
May highlights
24.5C - the highest temperature, recorded at Wairoa on May 16.
-8.8C - the coldest temperature, observed at Mt Cook Airport on May 31.
152mm - the highest amount of rainfall in a single day, recorded at North Egmont on May 11.
187km/h - the highest wind gust, observed at Akitio, Tararua, on May 21.
The six centres
Auckland was the warmest, Tauranga was the sunniest, Dunedin was the coldest and driest, Hamilton was the wettest, and Wellington was the least sunny.
Sunshine hours
Of the available, regularly reporting sunshine observation sites, the sunniest four locations over the first five months of 2018 so far have been wider Nelson (1069 hours), Marlborough (1068 hours), Hawke's Bay (1042 hours) and Canterbury (1031 hours).
