MetService has issued severe thunderstorm warnings for several parts of the South Island.
The severe warnings cover Selwyn, Ashburton, and Christchurch city as well as Timaru and Mackenzie.
A severe thunderstorm warning had been in place for Central Otago, Waitaki and Dunedin City but has now been lifted.
Thunderstorms are expected to be accompanied by very heavy rain, large hail and frequent lightning.
"Very heavy rain can cause surface and/or flash flooding about streams, gullies and urban areas, and make driving conditions extremely hazardous," MetService said.
"Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, and make driving conditions hazardous."
Meanwhile, a thunderstorm watch is in place for Christchurch, Canterbury Plains, Canterbury High Country, North Otago, Central Otago, Southern Lakes, Dunedin, Clutha, Southland and Fiordland.
MetService said thunder could be accompanied by very heavy rain and large hail.
The watch was expected to be in place until 9pm.
"A cold and unstable air-mass affects the South Island. For much of Fiordland, Southland, Otago, and Canterbury from about Christchurch southwards, there is a moderate risk of thunderstorms this afternoon and evening," MetService said.
"There is also a moderate risk that some of these storms may become SEVERE between 3pm and 9pm today (Saturday), which could produce localised downpours of 25 to 40mm/h and/or hail larger than 20mm.
"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."
Poor visibility and surface flooding could make driving difficult, and large hail had the capacity to "cause significant damage to crops, orchards, vines, glasshouses and vehicles".
"Should severe weather approach or if you feel threatened, take shelter immediately."
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