For locations under a heat alert, MetService advised people to keep hydrated, seek shade, check in with vulnerable people and look after pets and livestock.
A cold change was expected on Friday and Saturday, Wotherspoon said, with a cold front moving up the country.
It would see temperatures drop in the South Island before picking up again early next week.
MetService said Alexandra and Taumarunui were forecasted to go from highs of 29C on Wednesday to just 18C and 19C respectively on Saturday.
“For those planning an extended weekend ahead of Waitangi Day, early indications suggest, aside from the occasional shower in Fiordland, a ridge of high pressure is poised to bring settled weather on Monday and Tuesday for the rest of Aotearoa New Zealand.”
On Tuesday, Tauranga Airport was the hottest location, recording a maximum temperature of 30.4C.
Twizel hit 30.1C.
Springs Junction had the coldest minimum temperature, at 5.7C.
Fire ban season in Canterbury
Canterbury will move to a prohibited fire season from midnight on Wednesday.
Fire and Emergency said that meant no open-air fires were allowed and all fire permits were suspended.
District Commander Dave Stackhouse said there have been several significant fires in Canterbury recently, and hot, dry, windy weather was expected to continue for the next two months.