A near-perfect recipe for a chilly morning is brewing for Wednesday as plummeting temperatures are already affecting some areas, according to NIWA forecaster Chris Brandolino.
Cold temperatures chilled some areas on Tuesday morning as a southwest flow struck the country and left frost in its path.
Brandolino told The AM Show that preliminary temperature readings for Tuesday morning showed the chilliest locations include Mt Potts on -7.3C, Clyde on -3.7C and the Desert Rd which hit -2.3C.
But the full brunt of the wintry blast may still be on its way, and is expected on Wednesday.
"Tomorrow morning looks like it will be the coldest morning when you look at New Zealand as a whole."
Previously Queen's Birthday had been one of the coolest days, at least at Niwa's station at Auckland's Western Springs which saw a chilly morning at 1.7C, Brandolino said.
"Tomorrow morning will come close to that, at least at Western Springs, I think in the CBD, not quite as cold but still chilly.
"Watch out for frost when you are driving because some of the roads will be slick as you are driving around.
"Hamilton tomorrow morning can be down or below 0C, Auckland on 1C or 2C. Down in the South Island some very cold temperatures."
Brandolino said the upcoming Winter Solstice on June 22 was contributing to the cold mornings.
"Recipe for a very cold morning: You want long nights, we pretty much have that, the longest night of the year are approaching the solstice.
"[You need a] cold air mass, this is coming from deep in the south, dry air mass, and we have light wind, high pressure.
"The only thing we are missing from the critical cold list is a fresh snow pack. A fresh snow pack really goes a long way because that allows for maximum radiational cooling - basically all the heat goes into space."