New Zealand's southern-most city, Invercargill, is having one of it's hottest days on record - with the temperature hitting 31.1°C this afternoon.

It's only the sixth time since 1993 the temperature in the city has exceeded 30°.

The hottest temperature Invercargill has ever experienced was 32.2°, but that was in 1921.

NIWA Weather tweeted that since 1905 there have only been 14 times in January that Invercargill temperatures have only reached or exceeded 30°.

Big time heat Sunday with max temperatures near or above 30°C over lower South Island. Since 1905, Invercargill temps have only reached/exceed 30°C 14 times in January. Going for number 15 & 16 Sunday & Monday. PS Watch fluid-like movement of the colours.
~Chris pic.twitter.com/j05n3FXbJf

— NIWA Weather (@NiwaWeather) January 13, 2018

A MetService meteorologist, Sarah Haddon, said many places in the lower South Island have had a scorching day.

Balclutha, Lumsden, Gore and Alexandra have all experienced temperatures of at least 30°.

"We have a high just sitting to the east of the South Island, as well as a low sitting in the Tasman Sea, so between those two features we have a northerly flow pulling a lot of tropical moist warm air, so that's lying over New Zealand bringing those temperatures up."

Ms Haddon said it was likely to be just as hot tomorrow, but things would cool down a bit later in the week.

It has been a #scorchingsummer️across New Zealand so far! Check out the Dec-Jan temperature difference from average...

The south is turning up the heat today (max temps through 2pm)

Otama 32.4
Waipahi 32.4
Tuatapere 31.9
Wreys Bush 31.8
Middlemarch 31.8 pic.twitter.com/XbALHQlsoO

— NIWA Weather (@NiwaWeather) January 14, 2018

MetService said the temperature would drop to the low 20s later in the week.

A low approaching slowly from the Tasman Sea is set to spread rain and strong northerly winds over most of New Zealand from the middle of this week. Today's satellite image shows the system lying in wait to the west of the country. Keep up to date at https://t.co/Yjbq0jxdqz ^SG pic.twitter.com/VRCCSFp7xy

— MetService (@MetService) January 14, 2018

Meanwhile, MetService is forecasting thunderstorms about the central and southern North Island up until about 9pm today.

MetService meteorologist Stephen Glassey said inland areas of the lower North Island were likely to get more thunderstorms this afternoon and evening.

"It's a high humidity air mass over New Zealand so it is warm across the whole country and that's one of the ingredients for thunderstorms."

The MetService said more thunderstorms were likely to keep disrupting summer, because the ocean surrounding the country was warmer than usual.

Yesterday warnings were issued as thunderstorms moved over parts of the lower North Island, causing heavy downpours.

Severe weather forecaster Andy Downs said the warmer sea was adding moisture to the air.

He said more storms were likely before summer was over.

"All in all summer is going to be a bit of a mixed bag. Obviously though the one good news is it should be staying fairly warm but of course potentially quite unstable."