South Islanders enjoying a hot summer's day today should brace for a short-lived but even hotter day tomorrow.
MetService is predicting temperatures in the 30s to sweep across the south from Dunedin to Picton.
Blenheim was poised to smash December records with temperatures rising to 36C tomorrow.
The scorching day would mean Dunedin should reach 30C and Timaru and Christchurch 33C.
Today was a "quiet" day with no rain expected for the North Island.
Aucklanders are enjoying a sunny day outside with a temperature of 22C.
It is fine in Christchurch today with a maximum of 20C, but Cantabrians should expect 33C tomorrow.
The sizzling weather will also sweep through the eastern parts of the North Island with Napier reaching 27C, Gisborne 28C and Masterton 29C.
Warm air from Australia sweeping through the east across New Zealand today and tomorrow is bringing the high temperatures, said MetService meteorologist Tom Adams.
"On Monday strong northwesterlies develop ahead of an approaching front, adding a Foehn heating effect to areas east of the Southern Alps," he said.
Foehn wind is a hot and dry wind that blows from the mountains downwards, creating the hot weather. However, the heat will be short-lived, meteorologist Karl Loots said.
"Unfortunately tomorrow night a cold front will approach the country."
It will spread on Tuesday and temperatures will drop.Rain is expected to spread from the West Coast, bringing showers to the North Island on Wednesday.
Although Auckland and Wellington will miss out on tomorrow's sizzler, both cities will remain mild with temperatures reaching 22C and 21C respectively.
Auckland will continue to be settled on Tuesday with cloudy periods.
"There will be late showers with westerlies changing to southerlies."
Wednesday is expected to be fine with a few showers in the afternoon.
Wellington weather will be cloudy with strong or gale northwesterlies with a chance of showers on Tuesday.
Christchurch has periods of morning rain followed by a few showers on Tuesday.