Western areas of the South Island are likely to bear the brunt of this week's wet and windy weather, but temperatures will drop well below the summer average in many parts of the country.
Heavy rain and strong winds are expected in the centre and south of New Zealand.
MetService forecaster Mmathapelo Makgabutlane said the western South Island including the Fiordland region could have 150-250mm of rain throughout today and into tomorrow.
In Westland, thunderstorms may continue during the day along with strong wind and even hail.
Where's the rain going? ⛈— MetService (@MetService) January 18, 2021
This animation shows a band of rain moving up the country today with a very showery, cool airmass behind it.
Thunderstorms are possible for a lot of western areas https://t.co/BZWb7ZPiRd ^Lewis pic.twitter.com/gLbjEOtgrH
Heavy rain warnings are in force for Westland and the headwaters of the Canterbury lakes and rivers, while downpours are expected at the headwaters of the Otago lakes and rivers, Buller, the western ranges of Nelson, the Tararua Range and Mount Taranaki.
Strong wind is forecast in the Canterbury High Country, Marlborough, Wellington, Wairarapa and central Hawke's Bay, Fiordland, western parts of Southland and inland Otago.
Makgabutlane said temperatures in the South Island were starting to fall.
"From today, already we see those temperatures starting to drop over the southern parts of the South Island, and all the way into the southern lakes as well."
Wanaka and Queenstown would reach a maximum of 15C, well below the January average, she said.
"That continues into tomorrow and spreads to other parts of the country as well ... the lower North Island, even into the central parts of the North Island, we see those temperatures only reaching the mid teens into tomorrow. "
"Those cold temperatures continue into Thursday with the first signs of back to average temperatures only coming through by the end of the working week."
A southerly air flow behind a cold front is bringing cooler air from the Antarctic, with the drop in temperature partly responsible for thunderstorms.
"Those cool temperatures, especially in the upper atmosphere, cause quite an unstable atmosphere, bringing very favourable conditions for [the] thunderstorm activity."
Otago's Cardrona Alpine Resort has closed its summer activities after snow felall this morning. Hiking, mountain karting and biking cannot take place as the trails are covered in snow, with 18cm predicted to fall during the day.