"Coastal places in the South Island can expect showers laced with hail and a daytime temperature of just 6 to 8 degrees on Wednesday."
Auckland will also feel the chill, with temperatures in the city struggling to make it into the teens, he said.
"Those taking part in the Big Sleep Out overnight on Thursday will need to wrap up warm and don a raincoat.
"A showery southwesterly and temperature of 6 degrees is forecast, with the wind chill making the temperature feel like 0 degrees."
WeatherWatch forecaster Philip Duncan said the cold change was simply a matter of "winter finally arriving".
"For many places it's just going to put the temperatures down to where they should be [for winter], so it will feel cold, but most places won't be too severe.
"It's a cold blast, but compared to the weather we've had, it will feel worse than it is."
The cold snap will be good news for skiers itching to hit the slopes, with snow predicted to fall as low as 300m in some parts of the South Island.
Looking further ahead, July is set to be a wet and windy month once the polar blast passes, according to a WeatherWatch outlook.
A big rain event could drench the upper North Island in the second week of the month, also bringing rain and gales to the rest of the country.
"At the moment the general pattern looks like more rain for the west, and drier for the east, and temperatures look little more average [for winter]," Mr Duncan said.
"We've got more southerlies in July than we had in June, and they're colder southerlies as well, but because it's looking windy and wet that limits the chances of frost."
July will be a damp and cold month for most places, he said.
"We're not seeing a lot of high pressure, just one high crossing New Zealand in the first 20 days of July, that means you have to rely on mountains and ranges to block rain and cloud, because the rain and cloud is going to be coming at us non stop."
Read more:
• Winter blast could bring snow as low as 300m
Tuesday forecast for main centres:
* Auckland: Showers in the afternoon, turning heavy and with a chance of thunder, westerly winds and a high of 16C, falling to 9C overnight.
* Whangarei: Afternoon showers, turning heavy with a chance of thunder, a westerly wind and a high of 17C, dropping to 9C overnight.
* Hamilton: Heavy showers in the afternoon, with a chance of thunder, westerly and a high of 15C, falling to 6C overnight.
* Tauranga: Showers, some heavy in the afternoon, with westerlies and a high of 16C, an overnight low of 7C.
* Wellington: Showers, some heavy, with some fine spells in the afternoon. Northerly wind, and a high of 14C, dropping to 7C overnight.
* Christchurch: Fine spells with a few evening showers, light winds and a high of 13C, set to drop to 0C overnight.
* Dunedin: Showers develop in the afternoon, light winds and a high of 10C, falling to 2C overnight.
(Source: MetService)
July outlook:
* Upper North Island, including Auckland, Northland and Coromandel: warmer than average and probably wetter than average.
* West Coast of the South Island: wetter than average, with temperatures probably fairly average.
* Canterbury: drier than average.
* Lower South Island: average temperatures.
* Hawke's Bay, Wairarapa, and Marlborough: drier than average.
* The general pattern looks like more rain for the west and drier for the east, temperatures average, and more southerlies than in June. A windy and wet outlook limits the chances of frost.
(Source: WeatherWatch)
