A family driving in Ashburton got a close up view of a tornado.

A tornado has been spotted crossing part of Mid Canterbury.

It's understood to have hit about 3.30pm on Sunday.

James Busby said he was in the garden, in Mitcham, north of Ashburton, when he saw the "motion of the twister forming" in a neighbouring dairy farm and watched as it gathered momentum, picking up debris, wood and anything else in its path.

"It was a massive funnel ... a really big one, I've never seen anything like that in New Zealand before, about 40 or 50 metres wide.

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"It looked like it was bang on their property so I hope they're all right.

"It was really sudden, the sky went all dark and the twister came through then suddenly it was all over and it died away as it moved off towards the sea."

SUPPLIED One witness described the tornado as a massive funnel.

Kat Beckett was driving just north of Ashburton when she spotted a "massive" tornado whirling through the middle of a field.

It didn't appear to be in the path of any animals or property but was "moving quickly". She believed it was at least 100m to 150m tall.

"I've never seen anything like that before, especially not at this time of year. Never."

KAT BECKETT/SUPPLIED Kat Beckett said she'd never seen anything like the tornado before.

MetService meteorologist Brian Mercer said about half a dozen people had contacted them about the tornado.

Cold air was moving up from the south and mixing with northwesterly winds, which created a twisting motion in the winds that could lead to a tornado, he said.

"They can happen, but they're quite rare," he said. New Zealand's landscape lacked large open plains found elsewhere in the world, meaning any tornadoes that did occur did not last long.

On Sunday evening, MetService issued a Severe Weather Warning for heavy snow in inland Southland, Central Otago and inland Clutha.

Cold southerlies were expected to bring wintry conditions to the south of the South Island through Monday into Tuesday. Snow would lower to 400 metres, with "significant snow accumulations" expected above 800 metres, the warning said. Southern Lakes, eastern Fiordland and inland Dunedin were expected to receive a heavy dumping in higher areas.

"Heavy snow may disrupt travel in affected areas and could damage trees and powerlines. Cold conditions may cause stress for livestock."

A Heavy Rain Watch was also in place in coastal areas of Clutha, Dunedin and North Otago from 4am on Monday and a Road Snowfall Warning has blanketed Arthur's Pass, Porters Pass, Lindis Pass, Crown Range Road and Milford Road. The road warnings were expected to lift overnight.

AIMEE BLACKLER/SUPPLIED A photograph taken on Sunday at 3.50pm, five minutes north of Ashburton, showing the massive tornado.

STORMS LASH CANTERBURY

The wild weather comes after hailstones the size of marbles and whirling winds pelted properties in Mid and South Canterbury. A thunderstorm also lashed the region.

MetService meteorologist Tui McInnes said "substantial" hail between 20 and 30 millimetres in diameter started to pour down over Ashburton about 3pm on Sunday.

MARK EVANS/GETTY IMAGES A severe thunderstorm warning has been issued for areas around Waimate.

MetService earlier issued a severe thunderstorm warning and said the storms were expected to move northwards through Mid Canterbury during the afternoon and evening.

Observed lightning strikes suggest reported tornado in the Ashburton area was part of a thunderstorm ahead of the main cold front and thunderstorm activity.



It's not unusual for discrete thunderstorms ahead of a main line to be capable of severe weather & tornadoes.



~Chris pic.twitter.com/uoGiwStcTK — NIWA Weather (@NiwaWeather) November 18, 2018

Netherby resident Matt Banks said hailstones "about the size of a 10 cent piece" pounded his property and thunder and lightning could be heard in the distance on Sunday afternoon. The storm passed quickly, but there were some "pretty ominous clouds heading our way", he said.

DIANE KING/SUPPLIED Hail pummeled the lower South Island.

MetService warned that "large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, and make driving conditions hazardous".

"Thunderstorms usually don't last more than an hour before collapsing so it's passed its peak and moving offshore but they can trigger others further up the line."

Further storms were forecast for the surrounding areas overnight.