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Christchurch was sitting at 16degC at about 10.30am on Thursday, Dunedin on 15degC and Timaru 14degC.
MetService is forecasting temperatures will reach a high of 17degC throughout the day in Dunedin.
Cloudy periods, and a few showers clearing tonight. Southwesterlies, strong at times until evening.
MetService meteorologist Andy Best said the cool change has been brought about by a southerly making its way up the South Island.
"It is currently very windy in the south. A southerly change has brought quite strong winds to the Otago peninsula and is moving up towards Kaikōura.
"That southerly is bringing a stark change in temperatures. People should expect a cooler night down to about 5 or 6degC in Queenstown and also places like Ashburton and Timaru."
There is a moderate risk of thunderstorms in Christchurch and North Canterbury on Thursday afternoon and a low risk in the central South Island.
It is expected to be so much cooler, that snow could fall in Fiordland.
"We are forecasting snow down to 1000m on Thursday. There will also be some sleety showers on Friday."
It is a stark contrast to the weather in the south this week.
Temperatures reached a scorching 36.1degC in Blenheim and 35degC in Christchurch on Wednesday.
It followed an even hotter day on Tuesday with temperatures smashing records across Canterbury.
Christchurch broke its 1979 record for the hottest day in January with 36.1degC .
Other places in Canterbury followed suit with Ashburton reaching 39.2degC , the highest temperature for the area since records began in 2006.
Friday is expected to also be quite cold, Best said.
"We are expecting temps down to about 16degC in Christchurch and 12degC in Invercargill which we would expect to be about 19degC for this time of year."
Soaring hot weather over the last few days put Canterbury fire services on high alert.
However, Fire and Emergency spokesperson Bruce Janes said the 5mm of rain expected in Canterbury won't be enough to alter dry conditions.
He said a good dump of rain is not expected until late next week, meaning fire services will continue to remain on high alert.