It is considered a heatwave when temperatures are consistently 5C above average for five days.
It is a similar story further south with Timaru expecting to reach 28C on Monday and 34C on Tuesday.
Dunedin and Nelson are also in for some warmer days this week, reaching 30C and 26C on Tuesday respectively.
McKensie Country Weather NZ is reporting Twizel is already sitting near 25C at 9am on Monday.
The sunshine is not expected to last all week with a front expected to bring a cool change to much of the country from about Thursday.
It follows a bleak summer so far in Christchurch.
"People would have felt these long stretches of below-average temperatures over the Christmas period," a MetService meteorologist said.
The south was battered by thunderstorms, heavy rain and strong gales last week.