South Island storm warnings issued as up to 500mm of rain forecast

Warnings have been issued as a "significant winter storm" approaches the South Island, with up to 500mm of rain expected in the coming days on the West Coast and heavy snow forecast for inland areas.

MetService said a complex low and frontal system would become slow moving over the South Island on Wednesday, bringing prolonged heavy rain which could turn to heavy snow in inland Otago and South Canterbury.

The Westland District has been upgraded to an orange-level heavy rain warning, with 400 to 500mm of rain expected in the ranges and 150 to 200mm near the coast over 48 hours from midday Wednesday.

Late winter storm to bring heavy rain and snow down south - Watch on TVNZ+

Heavy rain watches are also in place for parts of Buller, Grey, Canterbury, Otago and Fiordland, with further warnings possible.

Heavy snow is expected to fall above 800m, with lesser amounts as low as 400m, prompting a heavy snow watch for the Canterbury High Country south of the Rangitata River and parts of Otago.

MetService warned the snow could cause travel disruption and damage to trees and powerlines, while cold conditions could put stress on livestock.

NZTA warned of significant travel disruption, with more than 30cm of snow possible on State Highways 8 and 80 in the Mackenzie Basin and South Canterbury, and up to 20cm on State Highway 73 above 700m.

"Accumulations at this level are likely to impede travel and NZTA may close sections of highway or introduce restrictions to keep people safe and avoid vehicles becoming trapped," an NZTA spokesperson said.

Heavy Rain Warning - Orange

Westland - 48 hours from 12pm Wednesday to 12pm Friday

Heavy Rain Watch

Grey and Buller south of Westport - 24 hours from 6pm Wednesday to 6pm Thursday

Headwaters of the Canterbury lakes and rivers about and south of Arthur's Pass - 5 hours from 1pm to 6pm Wednesday

Otago Headwaters - 8 hours from 10am to 6pm Wednesday

Fiordland north of Caswell Sound - 12 hours from 9am to 9pm Wednesday

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Days of prolonged rain will swell West Coast rivers before heavy snow closes in on inland passes later in the week.

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Heavy Snow Watch

Canterbury High Country south of the Rangitata River and Otago north of Arrowtown to Ranfurly - 39 hours from 6pm Wednesday to 9am Friday.