Sudden Stratospheric Warming (SSW) event is in progress over the south pole, with temperatures rising more than 40 °C (72 °F) above normal

SSW occurs when the temperature of the stratosphere (30 - 50 km / 18 - 31 miles above ground) over the South Pole rises by more than 25 °C (45 °F).

Such events are rare in the southern hemisphere, with only two in New Zealand in recorded times - one in September 2002 and the other in September 2010, NIWA meteorologist Ben Noll said.

After the SSW in 2002, New Zealand experienced its coldest October in 20 years with below-average temperatures covering much of the country and frequent ground frosts. In 2010 – which is classed as a minor event – a number of rainfall records were broken with well below normal sunshine and very cold temperatures in parts of the South Island.

"For up to about a month after the SSW, polar air masses, known as streamers, can break off from the weakened vortex and move towards New Zealand. It doesn’t guarantee unusual or extreme weather, but it can happen," Noll says