Sunday was Invercargill's hottest day on record, reaching an all-time high of 32.3degC.
MetService announced in a tweet that the official temperature reading at Invercargill airport had climbed to 32.3degC by 5:30pm.
The weather service said the reading was "likely a new record high temperature for that location".
Invercargill's previous hottest recorded temperature was taken in 1921, when the city reached a high of 32.2degC.
On Saturday, NIWA tweeted that temperatures of or above 30degC have only been recorded 14 times in Invercargill since 1905.
Invercargill mayor Tim Shadbolt told Newshub the unusually warm weather made a nice change to the city's normal climate.
"We're always being teased wherever we go about weather. If you arrive at a town and it's raining they say, 'Oh you've brought your weather from Invercargill, did you, Mr Mayor?' and have you on about it. So it makes a nice change to be able to skite about having too much sunshine."
Mr Shadbolt confessed to feeling nervous about the next month's weather forecast, because "Murphy's Law can strike".
"This year for the first time ever we transferred the Burt Munroe challenge from November, which is usually a wet, stormy month, to February, and we had the best November ever recorded.
"So we're feeling a bit nervous about this weather climactic thing that's happening at the moment."
He said Invercargill's new water purification plant meant the city's residents have been able to keep their gardens looking good in the heat - not that he's around to inspect them.
"The irony of all this of course is that I'm in supposedly sunny Nelson getting drenched."