MetService is continuing to warn parts of the South Island to be ready for severe thunderstorms and heavy rain on Thursday afternoon and evening, with a potential for "damaging hail" to fall in some areas.

A weather radar picked up sever thunderstorms near Waikouati just after 2pm, causing a severe weather warning for Waitaki and Dunedin.

The thunderstorms are expected to be accompanied by very heavy rain and hail.

A severe weather warning was released just after 2pm for Waitaki and Dunedin.

"Very heavy rain can cause surface and/or flash flooding about streams, gullies and urban areas, and make driving conditions extremely hazardous," MetService said.

"Hail can cause damage to crops, orchards and vines and may make driving conditions hazardous."

Between Ashburton and Palmerston and extending inland to around Fairlie and Kurow, large hail in excess of 20mm in diameter could fall between 2pm and 6pm, according to the forecast.

For those in mid-Canterbury, South Canterbury and North Otago isolated thunderstorms are expected, especially in eastern areas nearer the coast.

The thunderstorms will include 10-20mm/h of heavy rain and large hail.

They're caused by a combination of an upper cold trough, onshore breezes and a southerly wind change, MetService said.

MetService says the thunderstorms should die away in the early evening.

The North Island is largely escaping the bad weather today, although there is a low risk of one or two thunderstorms around Gisborne and northern Hawke's Bay in the afternoon and early evening.

Meanwhile, those who are going to be celebrating the new year can expect "mint" weather, MetService said.

"The first weekend of 2022 is going to be mint.Clear skies and light winds dominate for most on the evening of New Years Eve," it tweeted.

"Everyone will see warm temperatures and clear skies this weekend, a lovely way to start 2022."