It was a lot colder than that inland, with Ohakune recording a temperature of -6.7C on May 22, a record low. Waiouru recorded a chilly overnight temperature of -6.9C on one night.
Winds were light to nonexistent and tending easterly and northeasterly at that time.
Niwa said the easterly flow was caused by high pressure above and to the east of New Zealand and lower pressure in the southwest.
The month continued a drier than usual autumn for much of the North Island.
Whanganui Airport recorded 57.4mm of rain during May - less than the May average of 73mm, Makgabutlane said. Rain fell across three days on March 3, 4 and 5, with a northerly wind gust of 70km/h on May 3 as it arrived and a strong southerly of 83km/h on May 5.
Over in Hāwera there was a wind gust of 111km/h on May 3. A roof was lifted off a
cottage, trampolines were upended and power lines were downed, causing outages to about 150 properties.
Waiouru had the fourth-lowest May rainfall recorded since 1961, with only 39 per cent of its normal total.
Soils are drier than usual for the time of year, Niwa says. This year Taranaki so far has the second-highest sunshine hours in New Zealand, after Bay of Plenty.
One exception to the North Island's dry, sunny autumn was Wellington, where there was flooding in early May.
Northern areas of the North Island have been in such a drought that strict water restrictions were imposed in Auckland on May 16. The wider region got some heavy rain on May 31, with flooding around Coromandel.
New Zealand's average May temperature was 11.3C, which is 0.5C above the 1981-2010 May average. It is now 40 months since there has been a nationwide temperature that is below average.
