Christchurch is forecast to reach 29C while Masterton will reach 27C.
Both Auckland and Hamilton are forecast to reach 25C while Gisborne and Napier will reach 26C and 27C respectively.
Law said temperatures were nearing the threshold for MetService to issue a heat alert.
“It’s the combination of the hot days and the warm muggy nights which is the more concerning figure,” Law remarked.
MetService will issue an online heat alert when temperatures across the country pass certain thresholds, the top 1% of temperatures.
Other aspects, such as high humidity, are taken into account before issuing the alert.
Law said they would ideally only be triggered once or twice a year.
“What we have coming up is not that each day is going to be exceptionally hot; it’s the combination of high temperatures during the day and really warm overnight.
“It’s that average temperature which could trigger that heat alert for places like Canterbury.”
According to MetService, Auckland’s overnight temperature would be 18C tonight and Wednesday.
The forecaster attributed the high temperatures to warm air being dragged down from the tropics towards New Zealand.
“The warmer the air, it tends to have more moisture which adds to the muggy conditions,” he said.
“We won’t lose the heat of the day and we’ll keep the muggy nights as well.”
MetService has issued two heavy rain watches for the South Island’s west coast from overnight until late tomorrow morning.
Law said windy conditions would plague Buller and Canterbury.
In the North Island, warm air in the Hawke’s Bay could be enough to bring in rain.
“Perhaps some heavy showers, some heavy rumbles of thunder and even some hail through the afternoon,” Law said.
Sign up to The Daily H, a free newsletter curated by our editors and delivered straight to your inbox every weekday.
