The temperature has reached 21.7C in Timaru, the second-warmest July temperature of all time.
"After the very cold weather they've had around about a week ago, it's quite a change," MetService meteorologist Peter Little said.
It's not the first time Timaru has neared 20C in July. Last year two days, July 3 and 6, came close to the mark.
Little said the balmy temperatures were due to the "very, very strong" northwesterly flow over the country.
"There's a lot of moisture being dropped on the West Coast, and as the wind descends on the eastern side of the [Southern] Alps, it warms up."
The cities could expect temperatures up to 20C to last until Sunday, when a southerly change would bring cooler weather, Little said.
Niwa preliminary data shows today was one of the warmest July days on record on the South Island's east coast.
Christchurch - 22.0 degrees Celsius at Kyle St. The fourth-warmest day recorded in July for all Christchurch stations (the Airport, Christchurch Gardens and the Niwa building in Riccarton).
Timaru - MetService temperature from their website was 21.7C, its second-warmest July temperature all-time. Records go back to 1885 there.
Oamaru - MetService registered 19.4C which is equal to seventh warmest all-time. The warmest July temperature recorded there is 20.0C.
Dunedin City (Musselburgh) - 18.8C is the fourth-highest July temperature on record since 1947 - 20.0C is the warmest.
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