A rare atmospheric event played a part in last night's strong gusts, rain and snow in some parts of the country, and will bring more cold spells.
The Sudden Stratospheric Warming started a month ago in the atmosphere, about 30 kilometres above Antarctica.
It can weaken an area called the polar vortex, which usually locks harsh, wintry conditions close to the South Pole, and allows cold air to go north.
Ben Noll, a meteorologist at the National Institute of Water and Atmospheric Research (Niwa), said the country had observed colder than usual temperatures.
"We are below average, so we look at our temperatures as a difference from average for the time of year, and if we're below that average - that's the long-term average over the last 30 years from 1981 to 2010, and we're sitting below average, that tells us that there has been an unusual chill in the air," he said.
Mr Noll said cooler than average ocean temperatures also played a role.
"We have a couple more chilly snaps to go, another one coming as we go into next week, that's right before the end of September - the Monday and Tuesday timeframes.
"As we go later in the next week, in the Thursday frame, another cold snap is possible for New Zealand. It looks like we've got a couple more chilly waves to go," he said.
Mr Noll said there will be colder temperatures, lots and rain and snow for the South Island and possibly the central North Island next week.
"Perhaps as we go into the middle part of October, There will be some warmer temperatures unfolding around New Zealand, but first we'll have to go through some of those winter-like spells before that happens."