MetService meteorologist James Millward said the lowest Dunedin got to last night was a sweltering 25C. Oamaru was only just behind on 24.8C and Queenstown on 24.3C.
Even Invercargill was at 23C about 1am, but dropped to 13C by 7am as a southerly front arrived.
Millward said those hot overnight temperatures were driven by the foehn effect, as dry northwesterlies swept down from the Southern Alps.
The country's east coast would continue its hot run into the day with many spots into the 30s.
The upper North Island was looking nice too with fine weather and a high of 26C forecast for Auckland, and similar conditions in Whangārei and Tauranga.
"We could see some brief southerly gales behind the front with strong gusts all the way through to central New Zealand," MetService meteorologist Kyle Lee said.
"Places like Blenheim are expected to peak at 33C on Friday ahead of the front with the maximum temperature dropping to 21C on Saturday,"
"More significantly, the overnight temperatures are expected to drop for the country as well, which will make sleeping easier over the weekend."
Strong winds were lining up those in the south in the Canterbury High Country including the foothills, Otago, Fiordland and Southland.
MetService was also forecasting a heavy rain watch for the west coast of the South Island tomorrow morning.
Yesterday, Hurunui, Pukaki, and Culverden were the hottest places in the country, reaching temperatures of 37.3C, 35.5C, and 35.1C respectively.
Director of The Hogget restaurant and bar Holly Sterne said in nearby Hawarden they were used to warm temperatures but not right through the day.
"We're pretty known up here near Cheviot and where we are in Hawarden, we're pretty well known for getting these kinds of temperatures," she said.
"But not day on day and not from early in the morning right through until the evening, normally it's just a peak temperature, so yeah, this is a bit extreme for us."
Sterne said some locals had come down seeking a reprieve from the heat under their air conditioning and to enjoy a cold drink.
Meanwhile, further up the South Island near Blenheim, MetService had recorded parts of the road which had reached temperatures over 62C.
An official heatwave had been reached yesterday for parts of the country following five days with temperatures 5C above average.
MetService meteorologist Georgina Griffiths said inland Bay of Plenty, Marlborough, and parts of Canterbury, including Cheviot, Culverden and Tekapo were all in one of these.
Griffiths said a number of other locations that had experienced consistent heat for three or more days were also likely to record official heatwaves.
They included Pukekohe, Hamilton, Taumarunui, Taupō, Palmerston North, Wellington, Porirua, Nelson, Dunedin, Gore and Invercargill.
Friday's weather forecast
Whangārei Long fine spells, and chance of a brief morning shower. Light winds. High 26C, Low 17C.
Auckland Morning cloudy periods, then fine. Light winds. 26C, 18C.
Hamilton Morning cloud breaking to long fine spells. Light winds. 26C, 14C.
Tauranga Morning cloud clearing to fine. Westerlies developing evening. 25C, 17C.
New Plymouth Morning cloud breaking to fine spells, then clouding over again late evening. Northwest breezes. 22C high, 15C overnight.
Napier Fine apart from evening high cloud. Light northwest, but afternoon sea breezes. 30C, 17C.
Whanganui Morning cloud clearing to fine, then rain for a time at night. Northwest breezes. 26C high, 14C overnight.
Wellington Morning cloud then fine spells and gusty northerlies. However, rain from evening with strong southerly. 22C, 14C.
Nelson Cloud increasing. Some evening rain as northerlies change brisk southwest. 26C high, 13C overnight.
Christchurch Fine at first with strong northerlies. Afternoon rain with a cooler, briefly gale southerly change. Wind easing evening. 29C, 11C.
Dunedin A period of morning rain as strong northerlies change to a cooler, briefly gale southerly, then becoming fine and winds easing. 24C, 11C.
