Last month was the warmest May that has ever been recorded in Whanganui.

Figures released this week by the National Institute of Water and Atmospheric Research (Niwa) show an "exceptional" month.

Whanganui's average temperature for May was 15.9C - a whopping 2.8C above normal.

That's the highest since records began in 1937.


Also a record high for Whanganui was the average overnight mean minimum temperature, which was 3.2C above average at 11.7C.

Whanganui's mean maximum temperature (the high temperatures during the day) was 18.7C, the second-highest since records began in 1937. The average is 16.3C.

On May 4, Whanganui recorded its third-highest May temperature of 24C.

Whanganui was not alone in being unusually warm. Temperatures were above average at nearly every weather station in the country. The national average temperature for the month was 12.9C, which was 2.1C above normal.

Niwa said the unusually warm weather was caused by a combination of northwesterly winds along with warmer sea surface temperatures to the west of New Zealand.

May was a damp month as well, with most of the 136mm of rain falling in the second half of the month.