A cooler change began to move up the South Island yesterday, and it will spread north up the eastern coastline today, WeatherWatch said.
The North Island will see the heat and humidity combine for another hot day - but with added rain.
"Wednesday will be hot and humid over the North Island with the chance of severe thunderstorms in the afternoon or early evening through central areas - over the Central Plateau and neighbouring regions," its forecast said.
MetService meteorologist Stephen Glassey said subtropical air over the North Island would remain.
Even though the temperatures were expected to dip a degree in Auckland over the next three days, higher humidity would mean little relief.
"It's going to be pretty sticky. It may feel worse."
MetService issued a severe weather watch today for Bay of Plenty, Taupo, Hawkes Bay, Nelson Buller, Westland and the Canterbury high country for significant rainfall.
"Meanwhile, a moist northeasterly airstream should spread on to the North Island. This is expected to bring rain to many areas, but about the central North Island, localised convection could bring torrential downpours," it said.
"In the afternoon and evening there is a high risk of thunderstorms with heavy rain in Taupo, the south of Gisborne, Bay of Plenty, Rotorua and Waikato, the west of Waitomo, Taumarunui and Taihape and about the Hawkes Bay ranges," MetService said.
"During the same period there is also a high risk of thunderstorms with heavy rain in the west of Northland."
Forecast
• Whangarei - Showers, becoming heavy and thundery from the afternoon, a high of 26C.
• Auckland - Occasional showers today, possibly thundery in the afternoon, with a high of 27C.
• Hamilton - Occasional showers, possibly thundery in the afternoon, with southeast breezes, a high of 28C.