A freak hailstorm lashed the capital this morning - causing at least three minor crashes.

Two were on the motorway between Thorndon and Aotea Quay and one on Wallace St in Mt Cook.

Affected vehicles were not blocking traffic and there were no reports of injuries.

Weather Watch spokesman Philip Duncan said the hailstorm was unexpected and is part of a swift southerly change that has been "dropping temperatures everywhere".

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"Wellington just got clipped by that southerly that's been coming up from the South Island," said Mr Duncan.

He said the front was moving up the Hutt Valley and into the Wairarapa.

Freak hailstorms are caused by volatility caused by the presence of "big fluffy cumulus clouds", a mixture of hot and cold air and the right amount of moisture in the air.

"You have all the right ingredients that then send these raindrops up a lot further into the atmosphere where it's colder and then it freezes and then you start to get hail," said Mr Duncan.

Once you get hail you can also get thunder, he said.

"When the hail starts knocking around up there in the clouds it can produce static charges and thunderstorms so it's a possibility we might see an isolated thunderstorm in the lower North Island as well."

Wellington Police spokesman Sergeant Andy Dow has asked drivers to drive to the conditions and to put dipped headlights on.