Many places around the country are expected to reach official heatwave status today, while temperatures are expected to be at their hottest this week reaching 37C in North Canterbury.
In meteorological terms, a series of high temperatures can only be labelled a heatwave when they remain 6C above average for five days in a row - a point expected to be reached in many places around New Zealand today.
MetService said the hottest spot was likely to be Blenheim, which could expect a high of 34C, while Masterton could hit 33C.
Meanwhile Niwa's modelling last night predicted a maximum temperature of about 37C would be reached in North Canterbury, somewhere to the west of Kaikōura.
More about the hot weather:
- Weather Q&A with MetService meteorologists
- Feelin' hot, hot, hot: Heat updates day three
- The worst jobs in hot weather
- 'Five days of heat guys, we're only in day two'
- Ice cream vendors under pressure
The heat has broken several temperature records already: Levin at 32.6C topped records going back to 1895, Hamilton Airport at 32C surpassed records back to 1970, Kaitaia Airport on 31C beat records back to 1948, and Kelburn - the Wellington suburb where MetService is based - at 30.3C topped all its records since 1927.
Sweeping across from Australia, which has been facing even higher temperatures and wider problems, the swelter has prompted cautions from doctors in New Zealand, a total fire ban in some places - including Northland, where firefighters have been battling scrub fires, and water restrictions across the country.
It also shut down train services during the evening rush hour in Wellington on Tuesday, and has farmers facing the possibility of drought.
MetService said cooler weather will start moving up the South Island tomorrow and should reach the North Island on Saturday.
Keep up with our updates as the heat peaks today: