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

Eastern Rangitaiki plunged to -5.3C overnight as frosts gripped the North Island.

Road snowfall alerts include SH1 near Dunedin and key alpine passes.

Snow is expected to build up across parts of the lower South Island from Sunday morning, with a moderate chance of MetService upgrading its warnings.

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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