"So after a couple of warm days it's going to get significantly colder, through Wednesday and Thursday."
Temperatures in Invercargill will drop to around 6 degrees on Thursday, and hover around freezing at night, Ms Walsh said.
Even Auckland will see a temperature shift, with night-time temperatures dropping as low as 6-8 degrees later this week.
Heavy snow looks likely to fall in all the main ski centres from Wednesday onwards. Southern Fiordland, Southland, Clutha, Dunedin and Taihape can expect to see heavy snow, while northern Canterbury and southern Marlborough could also see some snow.
North Island ski centres are likely to see heavy snow down to 700m.
"There will definitely be some snow road warnings in the South Island later on this week," Ms Walsh predicted, adding it was "good news for the ski season, though".
The wintry blast will be characterised by cold showers, with snow, hail and cold southwesterlies.
It comes after a wet start to the week, which could see up to 90mm of rain fall in parts of Fiordland tonight.
A severe weather warning is in place for Fiordland and Westland, with strong gales and severe gale northerlies in the mountains, and heavy rain in western and northern areas as a front moves east across the South Island from the Tasman Sea.
"Tomorrow is going to be a pretty mucky Monday morning for Auckland and a lot of places around the country. We've got a front moving in tonight, and that's going to deliver pretty heavy rain to western parts of the South Island and also some western areas of the North Island as well," Ms Walsh said.
"But that should ease back in the afternoon, especially up north towards Auckland, and there should just be a few showers in the afternoon."
Rain and wind is also expected in other parts of the South Island, Taranaki and Wellington.
"So it's going to be a wet and windy start for a few people. It's looking quite unsettled after the weekend where it gradually settled down. We're back into the washing machine."
Main Centres Three Day Forecast:
* Auckland - Monday will see rain and a strong north-east wind, with a high of 17C, expect 11C overnight. Showers will continue into Tuesday with a high of 16C and 9C overnight, while Wednesday will see strong westerlies and showers, with a chance of hail. A high of 14C, and a low of 8C overnight.
* Whangarei - Rain and strong north-east wind will start Monday, clearing in the afternoon, a high of 18C, overnight temperatures will drop to 10C. On Tuesday expect showers, becoming more frequent, a high of 17C and 8C overnight. While Wednesday will see showers with some hail and strong westerlies, a high of 15C, and 7C overnight.
* Hamilton - Rain and gusty northerlies will start the week, with a high of 16C falling to 8C overnight, followed by showers and northwesterlies on Tuesday, with a high of 15C, dropping to 6C overnight. Wednesday will be showery with hail, and fresh cold westerlies, a high of 12C will plunge to 3C overnight.
* Tauranga - Rain and strong northerlies will clear by Monday evening leaving a high of 17C, dropping to 10C overnight, but Tuesday will see showers from the afternoon, and northwesterlies. Temperatures will hit 16C, and a low of 6C overnight. Wednesday will be showery with strong westerlies, and a high of 13C, dropping to 4C overnight.
* Wellington - Monday will start with rain and northerlies with gusts of up to 100km/h, easing by the evening, a high of 14C and a low of 11C overnight. On Tuesday showers will clear by the afternoon, with the temperature again hitting 14C with a cooler night at 7C, while Wednesday will see showers with northwesterlies turning into a cold southerly, a high of 11C falling to 5C overnight.
* Christchurch - Light rain on Monday becomes a fine day, with a strong northeast easing and a high of 16C, falling to 5C overnight. Tuesday will see a few showers developing in the afternoon, with light winds and a high of 13C, but dropping to 0C overnight. Temperatures fall on Wednesday, with showers and hail, and a cold southwesterly, a high of 8C plunging to -2C overnight.
* Dunedin - Early rain will clear on Monday, with a high of 14C dropping to 5C overnight, followed by showers on Tuesday afternoon and light winds, a high of 10C will fall to 3C overnight. Wednesday sees showers with hail and cold southwesterlies strengthening, a high of 6C drops to 3C overnight.
Source: MetService.
