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Dunedin has not seen a December day this hot since records began over 50 years ago, soaring to a high of 34.6degC this afternoon.
That beats the previous record set on December 8th, 1990, when the airport recorded a high of 32.2degC.
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Around 2.30pm it was the hottest temperature in the country and was topped only by Ashburton at 35.2degC for hottest of the day.
The Dunedin City Council has warned that the high temperatures coupled with strong northwesterly winds will push the fire danger to extreme in the region.
All fire permits have been revoked across Otago, and anyone having recently lit a fire is asked by the Otago Rural Fire Authority to check that it is fully extinguished.
On 8 Dec 1990, #Dunedin Airport set a record temp for December of 32.2C. It has reached 32.7C there this hour.^CF— MetService (@MetService) December 20, 2015
Dunedin was the hottest place in the country for much of the morning as a heatwave began its spread across the eastern coastline of New Zealand.The heat in Christchurch reaching a sweltering 36.1degC this afternoon.
The heat in the city was hot enough to break the previous record of 35.4degC measured in December 1975 at Christchurch Airport.
It was also the hottest temperature of the day nationwide.
Christchurch and Dunedin were two of a handful of cities to smash temperature records which had been set previously in the month of December.
Timaru hit 34.1degC during today's mini-heat wave, while Ashburton hit a scorching 35.2degC earlier this afternoon almost reaching the town's December record of 36.3C set in 1971.
Other centres in the South Island also felt the heat with Oamaru reaching a high of 31.9degC and Blenheim 25.7degC.
One Canterbury resident, Ray, told Newstalk ZB his temperature gauge clocked 40degC - and the heat hit him when he stepped out on to his driveway.
Another man, who was driving between Christchurch and Timaru, said the temperature gauge on his car told him it was 37C outside.
He said he was driving past the Synlait factory near Dunsandel and didn't want to stop or open any car windows.
Warm air from Australia sweeping through the east across New Zealand and is bringing the high temperatures, MetService meteorologist Tom Adams said.
"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 [on Monday] 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.
- additional reporting NZME