More of the same is expected today and tomorrow, with just a light sprinkling of rain possible in the morning for Auckland, the Coromandel and the Bay of Plenty. Light cloud and gentle breezes will also take the edge off the heat for most of the North Island.
MetService meteorologist Sarah Haddon said the blast of heat was caused by a high sitting to the east of the country.
Haddon said it was unusual for the high to stick around for several days and cause such a prolonged increase in temperatures.
"A heatwave is when we expect temperatures five or more degrees above the usual, for five days in a row," she said.
The heatwave wasn't gracing the whole of the country but most regions were experiencing warmer weather, Haddon said.
Despite the heatwave, Invercargill residents have been urgently asked to save water for the next two days after an explosion at a pumping station, which severely injured a worker.
Council water manager Alister Murray said he was very conscious people would normally use more water than normal due to the heatwave but asked resident to make "strenuous efforts" to conserve water.
Metservice said a rise in temperatures when paired with cooler sea temperatures will also likely create sea fog in southern areas like Otago.
Weather Watch says a low in the Tasman Sea will slowly approach New Zealand towards the end of the week, bringing a heavier dose of rain and a drop in temperatures.
Heavy rain will mostly affect the West Coast of the South Island.
