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.
Do you care about mass surveillance?Related story: What Christchurch thinks of spy revelations
• Newsroom 03-943 2827 or email email@example.com
• Classified Ads: 03-3778778 or email firstname.lastname@example.org
• Display and Online Ads: 03-3648285 or email email@example.com
• General inquiries: 03 379 0940
• Subscribe to The Press
• Deliveries, subscriptions, holiday stop/starts: 03 364 8464, or email: firstname.lastname@example.org
• No paper or holiday stop/starts: action online
• Buy a photo
• Newspaper subscribers - register for the digital edition
• Make press.co.nz your homepage
Read Press extras for Avenues, fashion, home design and weddings
Births, weddings, engagements
Death notices and in memoriam