A MetService spokesman said the thunderstorms would bring rain with intensities of up to 25mm per hour across the Clutha and Southland areas between 2pm and 9pm today.
"There is a risk some of these thunderstorms could become slow moving and may become severe, producing localised downpours of 25mm to 40mm per hour.
"Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips.
"Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."
A Severe Thunderstorm Watch has been issued.
For information on preparing for and keeping safe during a storm, see the Civil Defence Get Ready website.
The southern warnings come as rain, humidity and warm temperatures are in store for much of the country today but better weather could be on the cards as we approach Christmas.
A warning is still in place today for the Coromandel Peninsula, which could have a deluge of up to 140mm of rain about the ranges, causing streams and rivers to “rise rapidly”, MetService said, with surface flooding and slips possible.
There are also rain watches in place for Gisborne ranges north of Tolaga Bay and Bay of Plenty between Tauranga and Whakatāne including Rotorua.
Everywhere else around the North Island can expect a wet morning with rain easing to showers throughout the day.
Overnight the rain has also made its way down to the South Island and thunderstorms are forecast for Invercargill, with belts of hail possible this evening.
There is a heavy rain watch in place for Tasman northwest of Motueka until 3pm today.
Rain in Christchurch and Nelson is expected to ease by this evening while afternoon showers in Queenstown could turn into downpours of heavy rain.
Niwa said the unsettled weather will stick around for the rest of the week as a big low pressure weather system spins “its tyres over the Tasman Sea”.
“This will keep things pretty unsettled around our shores, siphoning moist air down from the Coral Sea.”
MetService reported that the country will continue to get warmer than average temperatures for this time of the year.
Today’s temperatures range from lows between 11C and 19C to highs between 20C and 25C.
Kaitaia, Twizel, Wanaka and Invercargill are all expected to be the hottest areas today with forecast highs of 25C.
Yesterday, Waiau in North Canterbury was the hottest, recording a sweltering high of 29.1C.
MetService said Christchurch Airport was not too far behind with a high of 27C.
Meanwhile, a sub-tropical air mass is expected to continue to create humid conditions for the South Island into the weekend and into next week for the North Island.
Kiwis will be hoping for a break in the unsettled weather to celebrate Christmas and meteorologists say finer weather may be on the way.
“While we can’t yet wrap a gift of sunshine for Christmas Day, high pressure looks to slide toward New Zealand leading up to it, a favourable signal overall,” Niwa tweeted.
- additional reporting NZ Herald
