Meteorologist Mmathapelo Makgabutlane said the latest warning is for Auckland, Taupō, Lower Hutt and Blenheim.
"This alert is for today and the next few days coming," she said.
Makgabutlane said other affected areas include the central North Island, Central Otago and the Southland region.
The alert can also act as a public health measure, and a reminder that people should look after themselves when out enjoying the sun.
"We would definitely be recommending ... for people to drink plenty of water; where possible, staying out of the heat; making efforts to keep cool and also looking out for and keeping an eye on people who are a little bit more vulnerable to the heat," Makgabutlane said.
Lower Hutt's outdoor pool, McKenzie Baths, had its busiest day of the season so far.
Assistant manager Eli Davidson said he had to call in more lifeguards within half an hour of the pool opening.
The pool's capacity is 250, and today there was 180 people at the pool, with 103 of them swimmers.
MetService is continuing to monitor other centres in case they need to be added to the alert.
''Possibly the Bay of Plenty could see a heat alert being triggered in the next couple of days. We've got temperatures in the upper 20s and maybe even touching 30 degrees later on this week.
"We're also looking at places in the Waikato, so Hamilton could possibly be in there as well [with] temperatures in the 30s for the next couple of days."
Makgabutlane said temperatures are expected to drop down again with a southerly change later on in the week.
"Midweek we have a bit of a southerly change coming through which serves to cool things down just a little bit, with temperatures towards the lower and mid-20s for a lot of the country."
But she said not everyone will be seeing a reprieve quite so soon.
"Further north, it looks like [the] cooler air doesn't quite reach by midweek, maybe by next weekend the temperature will drop down again there."
