The hottest spots today are forecast to be inland, with Taumarunui and Te Kuiti both looking at a maximum of 34C.
Yesterday the warmest places around the country all occurred out east, with Napier and Blenheim both reaching 35C to top the lot.
MetService meteorologist Angus Hines said today the weather situation was slightly different, with the high pressure squirting out to the east of New Zealand directing an easterly onshore flow into some eastern areas.
An easterly flow means some eastern spots will be 5-10 degrees colder than yesterday (but still in mid 20s!, while western spots will be 5-10 degrees warmer than yesterday! All forecast temps at: https://t.co/Yjbq0jxdqz ^AH pic.twitter.com/90Q6DvsgBl
Hines said the same weather from the long weekend would continue throughout this week.
Hot air would continue to move over the Tasman Sea, unloading a barrage of sticky weather.
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.
This came after a roasting Sunday night in the Nelson region, when Richmond set a new record for the highest minimum temperature at 24.3C.
Niwa said the heatwave was expected to peak on Wednesday and Thursday, with maximum temperatures of 36C plus possible.
When will New Zealand's heatwave end❓
Heat is expected to peak on Wednesday-Thursday with maximum temperatures of 36˚C+ possible, then a southerly change will cool things down drastically for Friday (south) and Saturday (north). pic.twitter.com/J5Epvigkdv
But acceptability would depend on factors including size of classroom, humidity level, availability of heating and air conditioning, and the subject taught.
Those experiencing uncomfortable workplace temperatures could the issue with their workplace health and safety representative or call the local PPTA field officer.
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]."
"That brings calmer, settled conditions, which allow the sun to nudge those temperatures even higher."
The hot weather could be defined as a heatwave because a number of places country-wide were forecast to meet the threshold, 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 are on track for a heatwave as are Blenheim, Kaikōura and Ashburton in the South Island.