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Twizel and Lake Pukaki woke to pre-dawn temperatures of about -8degC.
Temperatures plunged overnight in the lower South Island because of a powerful high-pressure system over New Zealand.
The outskirts of Dunedin dropped to -7degC around 6am.
According to MetService Mosgiel was the coldest place in the country at 9.30am sitting at -5.8degC.
Christchurch, Wanaka and Queenstown also hovered around -5C at 6am.
Frosts and fog could linger around many areas in the South Island but once those burn off, the south can expect a fine day.
Niwa said Middlemarch, outside of Dunedin, had New Zealand's coldest June temperature since 2015 after it hit -12.3degC today.
Central Otago District Council said this morning the region had another frosty start to the day.
"Mainly clear skies with the odd patch of fog, dry roads with temperatures sitting between -7°C and -1°C."
Danseys Pass Rd, from the gates of the Danseys Pass Hotel, was still closed due to snow and ice, it said.
Queenstown Lakes District Council said the cold temperatures has brought more ice on the roads.
"They are right at the limit of where CMA (the stuff sprayed onto roads to reduce the likelihood of ice forming) stops working so some areas have been gritted this morning as an extra measure.
"Significant ice has been found and gritted in the Kawarau Gorge, Kingston Highway, Queenstown Hill, Fernhill, Bobs Cove and Wilson Bay. Icy spots will keep forming until well after the sun is up so take it very carefully today."
Meanwhile, the North Island has much warmer temperatures - good news for this afternoon's sold-out match between the Blues and the Hurricanes at Auckland's Eden Park.
Current forecasts are for a few showers to clear the city early this morning, with cloud increasing during the day and northeasterly winds. A temperature of around 15C is expected for the 3.35pm kick-off.
Northland, Coromandel, Gisborne and Hawke's Bay can expect morning showers before fine spells in the afternoon.
The day will be mainly fine from Waikato through to Wellington, with some light rain possible around Taranaki from this evening.
Looking ahead, the current high will move east and offshore later today, with a milder subtropical northerly flow developing, forecaster Weatherwatch says.
By Monday night and into Tuesday, the milder winds will mean warmer-than-average overnight lows - meaning temperatures in parts of the South Island will move from several degrees below normal to several degrees warmer than normal within 48 hours.
- additional reporting NZ Herald