Up to 400mm of rain is forecast for northern Canterbury and Marlborough.

A heavy rain warning has been issued for coastal Otago - with a chance of it turning into a red warning - as several other parts of the South Island brace for heavy snow and rain.

Flooding, slips and road closures could disrupt travel and isolate communities.

Up to 400mm of rain is forecast for northern Canterbury and Marlborough.

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The Kowhai river breached its banks at 10.26am.

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Hundreds of Kaikōura households were forced to leave their homes amid flooding.