The National Institute of Water and Atmospheric Research says a sudden shift in temperatures high above Antarctica this month could bring icy 'Beast from the East' weather events to New Zealand next month.
In a release today, NIWA forecaster Ben Noll said there is potential for a 'Sudden Stratospheric Warming', or SSW, to take place next week.
An SSW typically is defined by the temperature in the stratosphere above the South Pole rising by more than 25 degrees.
Antarctica is usually circled by a ring of very cold, stormy weather - parts of which drift north and hit New Zealand from the west.
During an SSW, that flow can be reversed, which would lead to icy weather hitting New Zealand from the east - a much more rare phenomena.
"These events are rare in the southern hemisphere," Mr Noll said.
"There have only been two in New Zealand in recorded times: one in September 2002 and the other in September 2010."
A major SSW took place in the northern hemisphere in February of 2018, which led to a series of extreme cold snaps across Europe, with the event dubbed the 'Beast from the East'.
If the southern SSW takes place, it's possible that polar air masses could break off from the Antarctic region and impact New Zealand - probably during September.
"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," Mr Noll said.
"It doesn't guarantee unusual or extreme weather, but it can happen."
Mr Noll said NIWA's forecasting indicates plenty of unsettled weather during September and October, with frequent cold spells.
For a full forecast for your region, see our weather section here.