Heavy rain warnings have also been put in place for the Westland and Buller Ranges and parts of the Tasman district with up to 180mm expected to fall in the Kahurangi National Park.
Those in the area are being warned to watch for rapidly rising streams and rivers.
The forecaster said a cold front would sweep up the South Island on Thursday, then over the central North Island on Friday. This would bring heavy rain and cooler temperatures to the west and north of the South Island.
While the nation is experiencing relatively warm temperatures today, changes in the deep south tomorrow would see a dive to a wintry 0 degrees.
Metservice said there was potential for snowfall above 400m in Southland and Canterbury.
While Christchurch would enjoy a balmy 23C tomorrow, the mercury would plunge to a wintry 10C by day's end.
The latest round of wet weather comes just weeks after western regions of the upper South Island were inundated in by devastating floods last month.
Record-setting rainfall saw Nelson and Marlborough put into a state of emergency as rivers breached their banks and properties disappeared in floodwaters.
Homes around Nelson were evacuated with slips collapsing hillsides, and leaving properties ruined.
