New weather maps show the freezing Antarctic surge starting to numb New Zealand.
The maps, created by Majicweather.com, show the polar winds coming in off the ice shelves, bringing a nationwide temperature drop.
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"These maps give a very clear picture of the Antarctic surge moving up New Zealand and how it behaves much like a swell, or wave, moving into a beach," WeatherWatch says in a statement.
"The cold polar winds 'hit' the South Island today and 'roll' up the country tonight and through Wednesday, pulling more and more to the east as it does so - caught up in the enormous low-pressure belt that is not only dredging up this cold southerly - but also pulling it away from NZ as it tries to head north."
The maps show the colder southerly winds in blue and the warmer northerly winds in orange. The lighter the colours are, the stronger the winds are.
WeatherWatch warns the colder winds will bring single-digit highs, which will affect a number of regions on Tuesday and/or Wednesday.
"The South Island is impacted by the colder than average air and wind chills below zero are expected with showers, sleet and even some low-level snow on Tuesday," it says.
"Sea level snow is possible in Milford Sound while a few flurries will make it into Southland and Otago, possibly down to 100m.
"Dunedin City may get a few flurries on Tuesday night too and into Wednesday morning at the peak of this cold snap."
Fortunately, by the end of Wednesday, a milder Aussie westerly airflow rolls into the very lower South Island and spreads over the country on Thursday, restoring warmer temperatures.