Showers are likely to develop in the Wairarapa and inland areas from Taranaki down to Wellington this afternoon, with possible thunderstorms in the evening.
Today's temperatures are forecast to reach into the high teens or low twenties for much of the North Island, while a swathe of the South Island is on the verge of a heatwave that will last most of the week.
NIWA is predicting near-record temperatures in the south, with today ranging from the high teens through to the mid-20s.
"Sunshine and warmth will be the theme for most of the country as we head into the working week," MetService meteorologist Claire Flynn said.
"The warmest spots are expected to be in the South Island, with Alexandra looking to reach 26°C on Tuesday and Wednesday, and Invercargill reaching 25°C on Monday."
A heatwave is defined as five days in a row where highs are at least five degrees above average, Flynn said.
"With temperatures forecast to be in the mid-twenties, parts of Southland, and inland Otago and Canterbury are likely to experience a heatwave this week, due to the persistent warmth."
TODAY'S TEMPERATURES
WHANGAREI
Cloudy with showers, fine spells increasing. High 20C, Overnight 15C
AUCKLAND
Morning showers, fine spells in afternoon. High 20C, Overnight 13C
HAMILTON
Few morning showers, becoming fine. High 20C, Overnight 10C
TAURANGA
Chance of morning showers, then fine. High 18C, Overnight 13C
NAPIER
Partly cloudy. High 20C, Overnight 10C
WELLINGTON
Showers clearing by midday. High 17C, Overnight 12C