Hanmer Forest in north Canterbury has hit a record-breaking 38.4degC.
NIWA posted on Twitter that Hanmer Forest had hit the wildly hot temperature on Thursday - the warmest it's been since records began in 1906.
- An end in sight? 'Cold change' coming to end New Zealand heatwave
- New Zealand heatwave: Safety warnings as hot weather peaks
NIWA says it's also New Zealand's fourth-equal highest temperature for January on record, and the 18th-equal highest temperature for New Zealand in any month across the year.
Further north in Marlborough, fire crews are on alert as a worrying change to dry, northwest winds kicks in, increasing the fire risk.
The country has been embracing the warmer weather since it kicked in on Sunday.
And on Thursday, MetService confirmed to Newshub the hot week was officially a heatwave in some parts of the country.
Masterton, Blenheim, Hanmer Springs, Culverden, Kawerau and Lower Hutt have all had five days in a row with temperatures at least 5degC hotter than the average high for this time of year.
The hot weather is expected to cool on Friday, with Weather Watch warning of a 'Great Cool Down' that will see temperatures in parts of the South Island drop as much as 14degC.