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Niwa said Ranfurly recorded a minimum temperature of -9.1degC yesterday morning, colder than any time in all of June 2016 or June 2017.
The frost has been accompanied by low cloud and mist. The sun only returned to Middlemarch yesterday, after three days of thick fog.
The town had minimum temperatures colder than -5degC for each of the past four mornings, yesterday morning reaching a low of -6.1 degC.
Lauder reached -5.1degC yesterday morning.
The sun may have been shining early on, but soon the fog crept up, making visibility a challenge.
Poolburn School pupils were playing outside at lunchtime, striving to keep warm among the extreme cold and hoar frost.
"It’s the biggest [frost] I’ve seen in my four years here," principal Sarah Read said.
MetService meteorologist Ravi Kandula said Alexandra struggled to reach a high of -0.4degC, but no frost lingered for too long.
Mr Kandula said the weather could simply be attributed to the start of winter.
"It’s the cold air mass, clear air, clear skies and overnight cool," he said.
Temperatures are low again across the region this morning, Dunedin Airport one of the coldest places on -5degC at 7.45am. Wedderburn and Omakau were also hovering around -5degC, while Queenstown, Wanaka and Alexandra were all below 0degC.
The NZ Transport Agency says all main southern highways are open, but there are numerous ice warnings in place. Check highway conditions here