The weather service today issued the alerts for parts of the South Island, with the temperature in Blenheim expected to peak at 31°C, Nelson, Alexandra and Christchurch at 28°C and Gore to reach 27°C, while Napier in the North Island was forecast to hit 29°C.
MetService began issuing heat alerts for 46 towns or cities in 2021 when forecast temperatures were expected to be unusually hot.
Meterologist Mmathapelo Makgabutlane said today's warnings were the first for the 2024-2025 season.
The trigger for the alert came when temperatures tipped into the top 1% for that area.
"We are now in summer, and it has come out in full force. We've had hot temperatures for the last few days, especially in the eastern parts of the country, and that does look set to continue today.
"For the heat alerts, we're really looking at temperatures that are more than just your typical hot summer's day."
Makgabutlane said while some areas were already hitting 30°C during the day, it was the overnight temperatures contributing to the unusual highs.
"Those temperatures remain warm throughout and because of that, we currently have some heat alerts.
"The areas that we are looking at include places like Blenheim, Napier and Hastings for those hot daytime and hot nighttime temperatures."
The alerts were a warning to vulnerable groups - who were more likely to be affected by temperatures spikes - such as the elderly, very young, and those on medication, she said.
Metservice was anticipating a hot summer and would keep its website updated regarding heat alerts.
It recommended people in hot areas stay out of the sun and keep hydrated with plenty of cool drinks.
For those who were feeling unwell, it was important to rest and rehydrate.
