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Many regions could expect to be baked by temperatures heading above 30degC today.
However, the hot, sunny weather was expected to be interrupted with a few thunderstorms as the day goes on.
Shortly before 2pm, MetService meteorologist Kyle Lee said Wanaka had hit the peak temperature of the day so far, with 29.3degC. An hour later, the temperature had climbed to 32.3degC.
The popular tourist town yesterday was bathed in 35.2degC heat - the hottest day ever recorded there - smashing the previous record of 34.5degC measured in February 2005.
People in Central Otago have been told to brace themselves for temperatures of 40degC and potentially higher early this week.
Some of those working for earthmoving company Maungatua Contracting, which provides civil construction services in Wanaka, suffered heatstroke after toiling in the unrelenting heat last month.
While it will be another stinking hot week, project manager Kane Duncan's said the team had measures to combat the heat, even if some people learned the hard way about following advice to drink plenty of water.
Duncan said the company also provided sun protection and would set up a marquee for some tasks. Workers also stayed off high intensity jobs in the hottest weather.
And there was one benefit to starting at dawn to avoid the heat.
"Quite a few guys knock off early and go for a swim."
It's been a very hot summer - and a potential record-breaker, with January likely to be days away from being the warmest month in New Zealand since records began in 1909.
The heat arrived early and fiercely in November and had barely releasing its sweaty grip since. Early this week could be the fiercest of all - including the potential for the heat to reach 40degC.
MetService meteorologist April Clark said nor-westerly winds and clear skies would combine to bring hot weather to the eastern and inland South Island on Monday and Tuesday.
The southern part of the island, in areas such as Central Otago, would be hottest on Monday; on Tuesday the worst of the heat could move further north to areas such as inland Canterbury, Clark said.
After toppling its record with 35.2degC yesterday, Wanaka was forecast to reach 34degC on Monday and 33degC the following day.
The town is in the middle of an official heatwave, considered to be when the daily maximum temperature is at least 5degC higher than the average maximum for five consecutive days. Wanaka's average maximum is 24degC.
'We're in a desert'
Fellow Central Otago heat trap Alexandra was also set to roast, with 35degC forecast for Monday and Tuesday.
Breen Construction health and safety manager Chris Lambeth said they used shadecloth, wide-brimmed hats and crystal-filled cooling scarves and vests to combat the heat.
At Nichol's garden centre in Cromwell, 30km northwest of Alexandra, assistant manager Bev Campbell rotates staff to ensure none are left too long outdoors.
Hats, sunblock and ice-blocks also helped.
For the plants, it was just water, water and more water.
"We're in a desert and we're furiously trying to keep plants alive."
New Zealand's hottest recorded of temperature of 42.4degC occurred in Rangiora, north of Christchurch, in February 1973. The North Island notched up its highest temperature, 39.2degC in Ruatoria, on the same day.