MetService has issued orange heavy rain warnings for Banks Peninsula and the region north of Amberley, and the Kaikoura Coast and ranges from Wednesday.
There are also road snowfall warnings for Porters and Arthur's passes.
Christchurch and Canterbury south of Amberley are under a heavy rain watch, while there's a heavy snow watch for the high country south of the Rangitata River.
There are strong wind watches for the Buller, Grey, Westland and Nelson Lakes districts and the Marlborough Sounds.
MetService meteorologist Mmathapelo Makgabutlane said the weather system was "multi-faceted."
"From rain to wind to snow for some inland parts of Canterbury, and associated drops in temperatures."
Further north along the Kaikoura Coast and ranges the rain warning begins around midday Wednesday and runs until Friday morning, bringing around 150 to 200 millimetres of rain.
Makgabutlane said coming after a wet start to the year, the prolonged rain could lead to rapidly rising rivers, surface flooding and difficult driving conditions.
She warned parts of the South Island are in for particularly cold weather from late Wednesday into Thursday, and single-digit maximum temperatures are expected on Thursday with highs of around 8C for parts of inland Otago, while Canterbury should expect maximums of 9 or 10C.
Most of the wet weather should clear for Canterbury by Saturday, with one or two showers possible towards the end of the weekend, she said.
Parts of the country are also in for very windy conditions.
The western part of the South Island will see southeasterly winds, and some areas, particularly through narrower valleys, could see periods of "quite gusty southeasterlies ripping though there", Makgabutlane said.
People should take time to prepare, tying down any loose items.
The Marlborough Sounds and Wellington should also expect high winds.
"Winds are just part of life for Wellington, but we're seeing unusually windy conditions from late Wednesday into Thursday."
