Snow is expected to lower to 300m in those areas and heavy snow is possible above 500m.
Locals are being warned: "Accumulations may approach warning criteria".
Those in the North Island can expect to see the effects of this cold front by early tomorrow morning, with rain reaching the central part of the island by then.
MetService meteorologist Phillipa Murdoch said a band of rain that brought showers in and around Auckland and Waikato overnight was already weakening.
"We're not expecting too much for the rest of the day for Auckland.
"But for other places, yes - we are expecting more rain or showers that will become more widespread this evening around Waikato down to Wellington."
A high of 17C is expected in the City of Sails today, as well as a low of 10C overnight. Fine spells and a few showers are forecast and westerly winds are due to strengthen by this evening.
At 7am, the current extremes reported the warmest place to be was in Kaitaia, at 14.1C.
Alexandra was the chilliest spot in the country - with a temperature of 1.4C - the wettest was in Auckland's Waitākere (1.8mm of rain reported) and the windiest spot was in Whanganui, where wind speeds reached 48km/h.
Those in the south can also expect temperatures to drop significantly, with a maximum high of about 10C.
Minimum temperatures forecast today include 4C in Christchurch, 5C in Blenheim, 6C in Greymouth, the same in Dunedin and a teeth-chattering 1C in Queenstown.
ROAD SNOWFALL WARNINGS:
Drivers are also being urged to take heed of road snowfall warnings issued by the weather authority for roads including Lewis Pass (State Highway 7), Arthur's Pass, Porters Pass, Lindis Pass, Crown Range Road and Milford Road.
Up to 4cm of snow may settle on these roads - with a further 5cm to 10cm of snow due to settle on Milford Road (SH94) above 600m with lesser amounts down to 400m.
MetService meteorologist Andrew James said the wild weather would begin this afternoon in the South Island with a front plunging temperatures close to zero, dumping snow across the ranges.
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James said there was "high confidence" of severe west to southwest gales – over 85km/h, with gusts even higher – from Northland, Auckland to Taranaki and eastwards to East Cape early tomorrow. Exposed parts of Wellington, Marlborough and Kaikōura would also be affected.
Widespread showers and periods of heavy rain across the North Island were also forecast ahead of the front tomorrow.
The Auckland region would see southwesterly winds and some showers today, before both the wind and rain picked up rapidly on Wednesday morning.
The North Island's west coast would also see southwest swells in excess of 5m, James said.
"This is going to be quite a significant event. On land we are warning people to take precaution from the high winds, don't leave anything loose outside. And at sea there are going to be some very large swells."
Weatherwatch NZ said ocean swells could produce waves in excess of 9m in places, driven by "hurricane-force winds" at times.
"This makes for dangerous marine and beach conditions along the western side of the country, in particular the western side of the North Island which is more exposed to this set-up.
"There will be rogue waves that may go higher too - so a warning in particular to Aucklanders who fish on the western rocks: this isn't a safe week to do so.
"It's not a safe week for boaties anywhere in the eastern Tasman Sea area or Cook Strait."
WeatherWatch is also warning people about a "very windy Wednesday'' tomorrow for large parts of the North Island.
Gales exceeding 120km/h in some areas are forecast and which may cause power cuts and make for dangerous driving in some exposed areas.
"[It] may even cause damage to weak buildings or sheds''.
Today's weather
Whangārei 19C high, 11C overnight low. Fine spells and a shower or two. Westerlies strengthening this evening.
Auckland 17C, 10C. Fine spells and a few showers. Westerlies strengthening this evening.
Hamilton 17C, 8C. Showers clearing in the morning, returning evening. Westerlies, gusty in the evening.
Tauranga 19C, 9C. Morning cloud and the odd shower, then fine. Brisk, gusty westerlies.
New Plymouth 15C, 8C. A few showers, mainly evening. Westerlies strengthening in the evening.
Napier 20C, 8C. Fine, high cloud at times. Westerlies.
Whanganui 17C, 8C. Fine spells. Showers returning evening. Westerlies strengthening evening.
Wellington 15C, 6C. Morning showers with southerlies, some heavy with hail, then fine. Showers from evening, chance heavy, northerlies turning gale southwest overnight.
Nelson 17C, 6C. Mainly fine, but a few evening showers. Gusty southwest, easing evening.
Christchurch 15C, 5C. Fine at first. Rain from afternoon, possibly heavy. Southwest changing northeast afternoon, then strong southerly evening.
Dunedin 13C, 5C. Fine, but rain developing afternoon. Southwesterlies, becoming strong evening.
