MetService meteorologist Ciaran Doolin said a series of cold fronts was moving up the South Island today. They were already hitting southern areas and would reach Nelson by evening.
Heavy snow warnings are in place for Southland, Clutha, southern Fiordland, Otago and Banks Peninsula.
Otago and Southland had already had dustings to 500m and 300m respectively today, but snow could reach sea-level this evening in those areas, Mr Doolin said.
Above the Canterbury plains snow would to about 400m.
Mr Doolin said temperatures would also fall, the "usual suspects" being hit the hardest.
Inland areas around Mt Cook, Lake Pukaki and Twizel will shiver through a -6C night and some parts of Central Otago, around Cromwell, Clyde and Alexandra, will drop to about -4C.
State Highway 94 from Te Anau to Milford and SH6, the Haast Pass have reopened after being closed due to snow but there are restrictions in place, as there are for SH8, the Lindis Pass.
- Check highway conditions here
Conditions in the North Island aren't so bad today.
"We've got a low off to the north east of the country," Mr Doolin said.
"That's bringing a fair bit of [rain] to parts of Northland, northern Auckland and the Coromandel."
A heavy rain watch is in place for northern Auckland and Great Barrier Island and there is a heavy rain warning for Northland.
"It's expected to be slightly more intense up there although it looks like it's already started to ease."
Most places in the North Island are in for highs in the mid-teens today, with the coldest areas overnight still well above zero.
That changes tomorrow when the remains of the cold fronts lashing the south today cross the Cook Straight, bringing showers with them.
The mercury drops a couple of degrees in the North Island, while below Nelson and Marlborough in the south it doesn't get above 8C.
The coldest parts of the North Island will be in Wairarapa and Hawke's Bay tomorrow night. Masterton could be in for an overnight low of -1C.
