Big wet warnings for New Zealand's South Island

Forecasters issued a rare warning for New Zealand's South Island where heavy downpours are expected. (Jono Searle/AAP PHOTOS)

A rare red weather warning has been issued for Dunedin and surrounds in New Zealand's South Island with forecasts of over a month's worth of rain in 36 hours.

Sandbags have been made available for locals with reports some rivers have already burst their banks in the Otago city.