"The South Island will get a mix of fine spells and a few showers but it will also be mostly fine."
If Blenheim hits 35C on Monday, it will be the hottest December day for the town on record.
The fire service is advising Marlborough locals to be extra careful.
Stray fire sparks could be disastrous, Marlborough-Kaikoura rural fire authority fire chief Richard McNamara said.
"Mowing in the heat of the day is not a good idea," he said.
"Having your barbecue with solid fuel is not a good idea. Having a bonfire on the beach next to marram grass is not a good idea ... people may not be fully aware of the total fire ban."
Meanwhile, Nana's Cream Freeze owner Robyn Shannon said a 35C day in Blenheim would be "absolutely fantastic".
"We'll definitely be out and about in the township on Monday, hopefully giving people some relief if it does get that hot."
WeatherWatch.co.nz is forecasting similar highs in the 30s across Marlborough and Canterbury and in the east of Otago, Hawkes Bay, Wairarapa and Gisborne.
But Auckland and the west coast of the country would miss out on the 30C temperatures, head analyst Philip Duncan said.
"This hot weather is being created by New Zealand's geography so if you're in Auckland, Hamilton, New Plymouth, Palmerston North, Wellington or pretty much anywhere along the western coastline, you won't be experiencing any heat wave, but you will be experiencing slightly warmer, more average, temperatures than you have had over the past few days," he said.
The hot days weren't expected to last, with temperatures in the late teens to early 20s predicted for Christmas Day.
- additional reporting Lynley Bilby
