Overnight lows on Monday did not drop much below 20C across the country, MetService meteorologist Philippa Murdoch said. Auckland only got as low as 19.9C, while the top spot was Kaitaia on 21.7C.
WeatherWatch NZ analyst Philip Duncan said the highest temperatures on Monday were recorded in localised areas sheltered from the wind, not across the whole region.
A small spot north of Napier climbed to a sweltering 37C, he said, meanwhile, 36C was recorded in Marlborough Sounds.
Earlier Air New Zealand flights from Blenheim, Gisborne, Dunedin and Tauranga had been diverted.
Other flights are currently delayed or cancelled due to the low cloud that is not expected to clear until midday.
Due to low cloud, flights are currently delayed or cancelled at Wellington Airport. Low cloud is now expected to clear by midday. Air New Zealand have suspended all flights until midday. Please refer to your airline or check live flight information https://t.co/XIe75H0UI1pic.twitter.com/WKqE8R4XyV
The fog is supposed to burn off later this morning.
MetService said today would be another fairly settled day in Auckland with some cloud in the morning and even a spot of early drizzle.
"Once the sun comes up that's going to burn away quickly. Much of the afternoon and evening is going to be dominated by warm sunshine and looking at a warm temperature of about 29C," Hines said.
"Some of the warmer spots are more likely to be inland, we're forecasting for Taumarunui and Te Kuiti both looking at a maximum of 34C."
Those trying to beat the heat shouldn't be afraid to run plug-in fans in fear of running up the power bill, Consumer NZ technical writer James le Page said.
"Most plug-in fans have a relatively small electric motor [50-60W] so the usage isn't too bad," he said.
"A 60W fan running for four hours a day will cost around $2 per month with power costing 0.26c/kWh.
"If you have one running while you sleep for eight hours then it's $4 [a month]."
The hot weather can be defined as a heatwave because a number of places country-wide are forecast to meet the threshold, MetService meteorologist Tui McInnes said.
As defined by the World Meteorological Organisation, a heatwave is five consecutive days with maximum temperatures 5C above average for the month.
In the North Island, Tauranga, Whakatāne, Te Kuiti, Taumarunui, and the Hutt Valley were on track for a heatwave as were Blenheim, Kaikōura and Ashburton in the South Island.
MetService meteorologist Amy Rossiter said warm temperatures were expected over summer, but the length of the extremely warm period was unusual.
"Some parts of the South Island might only get six days over 30C in a year, and this week they could get five or six days in a row."
The warm air mass is forecast to stick around until Thursday before a front moves up the country on Friday, bringing cooler temperatures.
