New Zealand’s winter has gotten shorter by a month over the last 100 years, meaning “true winter” weather with very low temperatures, frosts and snow starts significantly later in the year and ends earlier.

Brett Mullan, from the National Institute of Water and Atmospheric Research (Niwa), crunched the temperature records from two 30-year-periods over the last century to see if New Zealand was experiencing the same contraction of winter weather noted in other parts of the world.

New Zealand’s winter starts at the beginning of June and runs to 31 August, a total of 92 days. In the South Island and some parts of the North Island winter weather usually means snow, frosts, “black ice” on the roads and temperatures as low as -7C overnight.

Mullan defined a winter’s day as one in which the daily average temperature was less than 9C. He examined temperatures between 1909 to 1938, and 1987 to 2016 from seven geographically representative regions around New Zealand.

Mullan discovered there was an average of 100 days per year between 1909 and 1938 when the temperature was less than 9C, compared with only 70 days per year between 1987 and 2016.

Winter has contracted about equally from both ends of the season, he said.

“I was quite surprised to get essentially the same results as the US – a decline of a month,” Mullan told the Guardian.

“The temperatures are rising, they are milder, they are also starting half a month later and ending half a month earlier, so they are becoming more concentrated in the centre of the June, July, August period.”

The temperatures are rising, they are milder, they are also starting half a month later and ending half a month earlier Dr Brett Mullan

Mullan’s study showed that winter as defined by temperature also varied from year to year and did not fall neatly within the three month southern hemisphere parameters.

Last year winter was “essentially” confined to July and August and June was unusually warm, while this year the reverse was true, and August was unusually warm.

The number of frosts throughout the winter months has also declined, Mullan found.

“I expected the general pattern because we know New Zealand is warming, but it interested me that it was a full month and was so close to the US experience,” he said.

“It will certainly affect things like kiwi fruit production because the north of the country is becoming too warm, they are gradually moving kiwi production south. ”

“Wintertime is also very important for famers when it kills pests. If it stays too warm pests can reproduce right through the year. There are benefits too. If temperatures are higher you increase the growing-degree days and crops can mature more quickly, but the natural system has evolved to live with the winter conditions and it is quite important.”

New Zealand temperatures would continue to increase and winters would continue to get shorter and shorter, he said.

The new Labour government in New Zealand has said climate change is this generation’s nuclear moment and has committed to several new policies to tackle the warming climate. They include setting a target of net zero for greenhouse gas emissions by 2050, establishing an independent climate commission and appointing a minister for climate change.