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The city is forecast reach 29 deg C on Monday followed by a scorching high of 36 deg C on Tuesday.
MetService meteorologist Alwyn Bakker said this is 13 deg C above the city's average temperature for this time of year.
"Christchurch is currently looking at three days in a row with more than 5 deg C above average."
It is considered a heatwave when temperatures are consistently 5 deg C above average for five days.
It is a similar story further south with Timaru expecting to reach 28 deg C on Monday and 34 deg C on Tuesday.
Dunedin and Nelson are also in for some warmer days this week, reaching 30 deg C and 26 deg C, on Tuesday respectively.
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.