"One of the interesting things is the warmest temperatures are at the bottom of the country in Southland and Otago," MetService meteorologist Stephen Glassey said.
"It isn't unusual in these northeast flows but it is counter-intuitive to what most people think."
Further north, Lower Hutt also hit 30.7C and Palmerston North reached 30C, Hamilton 27.8C and Auckland a relatively cool 27.5C.
It was slightly cooler in Tauranga and Whangarei which only reached around 25C thanks to the sea breeze.
WeatherWatch analyst Phil Duncan predicted that today could be even hotter for the south.
The North Island faces a different type of heat as humidity brings "thicker air", making lower temperatures feel hotter, Duncan said.
"Humidity levels aren't extreme but it's making temperatures in the mid to late 20s feel more like 30."
The weather is expected to remain fine and warm today in the south, although the North Island will see some cloud and possibly a few showers.
But rain and strong winds are on their way from tomorrow night or early Wednesday as a deep low-pressure system comes from the west.
Rainfall is likely to reach warning levels tomorrow and Wednesday for Westland, Buller, Nelson and western Marlborough, while severe gales could hit Wairarapa, Hawke's Bay and Gisborne on Wednesday.
Severe weather warnings are likely to be issued in the coming days as the system evolves.
The rain will bring little relief from the muggy weather — Glassey said it's likely to get more humid as the low approaches.
But cooler air will follow behind the low, according to WeatherWatch.