Christchurch reached 29 deg C on Sunday, but that high will drop to 12 deg C on Monday. The city had reached 11 deg C by 10am.
Kaikōura hit 25 deg C at the weekend, but will also reach a high of only 13 deg C on Monday.
The eastern side of the upper North Island could expect heavy rain and thunderstorms this afternoon. The Bay of Plenty, Coromandel and eastern parts of Northland could be hit with the downpours.
MetService meteorologist John Law told Morning Report that any time a norwester crossed the South Island, it created a "classic burn effect" and produced "incredibly warm temperatures".
However, the southerly change would have a significant impact on temperatures, with Christchurch and Kaikoura expected to reach only 13 deg C.
"That's cold for this time of year... and compare that to what we saw yesterday, it makes a big difference through there."
Further south, Queenstown would have a "great day ... it's not brilliantly warm but [there is] a bit more sunshine and a pretty decent story".
Conversely, the burn effect often brought heavy rain on the West Coast, he said.
"The Southern Alps are a fantastic divider across the South Island.
"On that western side you tend to get that cloud and wet weather, and when you get that air on the other side of the hills it's a lot drier, and moisture's come out as rain on the West Coast and that dry air as it comes down the slopes starts to warm a lot quicker, and that's when you get the warm temperatures coming through.
"The great thing about New Zealand and its topography and where we are in the world [is that] it all adds a really interesting aspect into the weather."
Watches and warnings over the weekend had expired, and the worst of the rain had cleared away to the east.