Yesterday's warm weather is set to continue for most areas today, but it won't last for long.
WeatherWatch head analyst Phil Duncan said temperatures in inland Canterbury yesterday peaked at 30C.
"It was a hot day today in many places. I think many New Zealanders might be saying: is this the first day of summer?
"[Yesterday was] the first day we've seen 30C across both islands popping up."
Temperatures will reach the low to mid-20s in most of the main centres today, but in Christchurch it could climb to as high as 31C.
It would be generally warm, with "pockets of hot weather".
"The heat isn't necessarily everywhere. Because of the nature of our mountains and ranges it affects the temperatures quite a lot. Places like Masterton might not be as hot as Hastings."
The weekend would be "back to spring" for many areas as a cold front swept across the country, Duncan said.
"What we're getting now is a little bit of a flashback of what we had earlier in spring."
Duncan expected temperatures in Christchurch to drop to 16C tomorrow as a southerly change moved up from Southland and then bounce back to 23C on Friday.
A wet front would hit most of the country later in the week, bringing rain and showers.
Unsettled "spring-like" weather would likely continue into December for most of the country, but "with hot days mixed in", Duncan said.
Auckland - Sunny spells, might be a bit of cloud around, 21C
Hamilton - Mostly sunny and dry, west to southwest breezes, 22C
Tauranga - Sunny and dry - 25C
Wellington - Sunny spells with brisk northerlies, 19C
Christchurch - Sunny, southerly change later in the day, 30C
Dunedin - Sunny, southerly change, 20