Weather: Thunderstorms, snow, possible tornadoes with deepest low in decade forecast in lower South Island

New Zealand's blast of stormy weather is forecast to continue into Monday for many parts of the country - with warnings that thunderstorms, strong wind gusts, heavy rain, hail, snow, and possible small tornadoes are all in the mix.

The cause is a deep low in the Southern Ocean dredging up air from the Antarctic, which is then merging with milder airflows to the north.

WeatherWatch explains this combination of mild and cold is creating the "perfect zone" for squalls, hail and thunderstorms along western New Zealand, especially around central areas.

"The coldest pulse of this system will move into eastern and northern parts of New Zealand later on Sunday and into Monday," WeatherWatch said.

"Air pressure in the lower South Island today is expected to drop to 967hPa in Bluff, which it hasn't done for at least a decade and is very low air pressure for New Zealand.

"More deep low pressure is expected to drag in wintry weather for the parts of the South Island across the coming week."

A severe thunderstorm watch has been issued until 6am on Monday for Northland, Auckland, Waikato, Waitomo, Taranaki, Kapiti-Horowhenua, Wellington, Marlborough, Nelson, Buller, Westland, Canterbury High Country, and Fiordland.

"These thunderstorms are forecast to bring heavy rain, hail, strong squally wind gusts, and possibly some small tornadoes about coastal areas," MetService warned.

"Between 10:30am Sunday and 6am Monday, some thunderstorms may become severe in western areas of the country from Westland to Northland, including Marlborough Sounds and Wellington.

"These severe thunderstorms could bring large hail greater than 20mm in diameter, damaging winds gusting greater than 11km/h, and possibly one or two damaging tornadoes near the coast."

And the impact of the storm is already being felt, with snow, hail and heavy rain hitting multiple parts of the country. Here are some of the best videos and photos: