The east coast of the South Island could be in for tornadoes, large hail and thunderstorms.

A severe weather warning issued by Metservice is warning South Islanders in Ashburton, Timaru and the surrounding areas that thunderstorms will likely hit over the next half hour.

These thunderstorms were expected to be accompanied by very heavy rain, large hail and possible tornadoes.

Along with the severe conditions came a risk of flooding and damage to both crops and homes.

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A severe thunderstorm watch had also been issued for Christchurch and Canterbury Plains.

Civil Defence advised residents in the affected area to take shelter, secure any loose objects around their properties and take care when driving.

The North Island hadn't escaped a drenching, either.

Severe weather warnings remained over the top half of the North Island as well as down in Taranaki.

Heavy rain warnings were in place for Auckland, Northland and the Coromandel Peninsula.

Periods of heavy rain were expected to equate to around 120mm of rainfall from Kaikohe northwards, while at least 80mm of rain was expected over the Auckland region.

The dousing was due to a "river of subtropical air" according to WeatherWatch meteorologist Philip Duncan.

Duncan said the unpredictable weather system made it hard to pin down which areas would be hit worst.

"You get weird weather from tropical rain, you may get showers and 100mm in the same area.

"That is flooding criteria and it may be localised within a small area," Duncan said.

The warm front was being fed by airflow from Fiji and New Caledonia, and slowed by a high to the east, which would spare eastern areas like Gisborne.

"It will produce heavy downpours, it is a vein of tropical rain feeding in from the New Caledonia area."