The hottest spots are expected to be Alexandra and Cromwell, where temperatures are expected to reach 32°C today, followed by 31°C tomorrow, Sunday and Monday.
Other places forecast to hit 30°C or more during that time are Wanaka and Mosgiel; Gore and Invercargill may also come close.
There will be some relief on Tuesday when easterly winds are expected to bring showers and rain, dropping temperatures to the mid-20s again.
Southern residents are being urged to apply plenty of sun screen and dress sensibly over the next few days.
MetService meteorologist Mmathapelo Makgabutlane said the higher temperatures were caused by a high pressure system over the South Island.
"Firstly, that means that it’s suppressing a lot of shower development and stuff like that, so showers are not really very likely for a lot of places at the moment.
"Then the second thing is, that high pressure is associated with subsidence or downward moving air, and as that air moves down, it compresses and as it compresses, it heats up to create warmer temperatures."
Under this high pressure system, large scale wind systems were also less of an influence on the day-to-day weather, she said.
"People would normally associate these kinds of warm temperatures with the foehn wind effect — when westerly winds heat up as they flow over the Southern Alps.
"But this time round, there’s very little wind which is helping."
Following the cooler temperatures in January, people had been "holding out" for a high pressure system like this one.
"It’s quite a balancing act because I guess the high pressure means drier weather as well and not much rain for farmers.
"But I think after a bit of a bummer summer, a lot of people will be welcoming these more typical summer temperatures.
"Let’s hope it’s not the last of it."
