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.
Today's forecast
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