Thursday morning is likely to be the coldest of winter for much of the country but, before that, snow is set to fall to sea level in the south and disrupt travel on Tuesday and Wednesday.
Frigid air directly off the Antarctic ice sheet is driving the drop in temperatures, dragging snow down to low levels in the east early in the week before clearing skies and lighter winds let overnight temperatures fall away.
MetService meteorologist Silvia Martino said the cold was striking even for the depth of winter.
"Even for our coldest month of the year, these are exceptionally cold overnight temperatures," she said.
"Whitianga and Hamilton are forecast to be almost ten degrees colder than average for this time of year on Thursday morning, at -3C and -5C respectively, and even the 'winterless north' is set to see frosty starts."
MetService meteorologist Lewis Ferris said the air behind the cold snap was colder than a usual front, having travelled straight off the ice.
"Instead of the air kind of coming out of the Southern Ocean and being a bit more of a sort of southwesterly, this area is coming up directly off the Antarctic ice sheet."
Hamilton's forecast -5C matches the city's August record, while Auckland Airport could be 1C, though Ferris said colder inland spots such as Ardmore and Whenuapai could dip below freezing.
"I would think temperatures below zero are possible Thursday morning for parts of Auckland," he said.
Ferris said the readings could stand as the coldest of the entire winter.
Interislander has cancelled all its Kaiārahi sailings on Tuesday, suspending services between Wellington and Picton for the day, while Bluebridge said large swells could delay or cancel its crossings on Tuesday and Wednesday.
MetService has also issued road snowfall warnings across several key routes. Snow is expected to lower to sea level on the Dunedin to Waitati Highway (SH1) early Tuesday morning, with 2cm to 4cm, possibly more, above 200 metres, while 3cm to 6cm could build about the summit of the Remutaka Hill Road (SH2).
Warnings also covered Lewis Pass (SH7), Porters Pass (SH73) and the Crown Range Rd.
Martino urged drivers to check regional forecasts and stay across road updates.
"Check the snow level in your regional forecast to prepare if you're going to be driving, particularly in Dunedin and Christchurch on Tuesday morning, and keep up with any updates on road conditions from NZTA Waka Kotahi and local council or emergency management."
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