Summer temperatures have rocketed across the South Island today as a nor'wester kicks into gear, with Alexandra reaching a staggering 29 degrees at lunchtime. It was 27 in Timaru at that time.

Philip Duncan at the Weather Watch Centre said Oamaru was just one degree behind on 26 degrees with Dunedin on 24. Blenheim has 23 degrees while Christchurch and Queenstown have 22. Gisborne is on 21 with Kaikohe, Hastings, Akaroa and Wanaka all on 20.

Auckland and Timaru both have 19 and are tipped to reach 20 this afternoon.

Mr Duncan said the temperatures are out of season. "Spring has only just started and to many scientists it is still technically considered winter for another week or so but temperatures today, right across New Zealand, are painting a completely different picture," he said.

"Today's daily highs may have some record breakers in it."

Christchurch experienced a "freakish" jump in temperature thanks to the nor'wester. Temperatures jumped from 9C to 21C in just 20 minutes, said's Richard Green.

While it was relatively common for southerlies to cut temperatures that much and that quickly, it was unusual to see it go the other way.

Dunedin Airport also had one of its hottest nights on record for September, as the temperature remained in the low 20s until 5am.

Eastern and central areas of the mainland could eye record highs for September as very warm air is being blown across the Tasman Sea, he said.

Both Queenstown and Christchurch reached 21C at 10.30am.

- additional reporting by NZPA