NIWA warns the sudden stratospheric warming (SSW) affecting Antarctica could be the "strongest warming event on record".

It says the event "continues to unfold" and has "set high-temperature records in the stratosphere". And it warns the SSW is already starting to affect New Zealand.

"Temperatures have been below average month-to-date in the South Island," NIWA says.

"[There is an] increased risk for southerlies in New Zealand into October."

A graph shows just how much stratospheric temperatures have increased compared to the average.