Aucklanders are being told to brace for a rocky 24 hours, with potential for strong winds and even gales in some places.
There are warnings of possible power outages, and drivers of motorcycles and high-sided vehicles are being urged to take care on the Harbour Bridge.
Vector's head of network field services, Marko Simunac, said the company was monitoring the weather and had crews ready to respond to any power outages.
"Our response crews and storm management teams are ready to respond and we are actively monitoring the latest weather advice and our own data on network performance."
Simunac said that if the weather caused any power lines to come down, Vector would remotely shut down power to that area as a public safety precaution until crews arrive on site.
Bad weather could mean this could sometimes take longer than usual.
"All lines should be treated as live at all times. If you see any damaged power equipment or power lines down, please keep a safe distance away and report it to us as soon as possible on 0508 VECTOR."
Around the country, forecasters were predicting low-level snow, strong winds and gales, thanks to the cold-weather system moving over New Zealand today and tomorrow.
Temperature highs of between 14C and 17C are forecast for Northland, Auckland, Waikato and Bay of Plenty and between 9C and 13C in the western parts of the North Island.
Highs of between 13C to 15C are on the cards for Marlborough and Nelson, 10C to 13C in Canterbury, 8C to 10C on the West Coast and 7C to 8C in Southland and Otago.
MetService has issued a number of severe weather watches for heavy rain, snow and gales; as well a severe thunderstorm watch.
"Strong wind watches remain in force for Northland, Auckland, Coromandel, Waikato, Bay of Plenty, Hawke's Bay south of Hastings and the Tararua District; while Southland, Clutha, Dunedin and Banks Peninsula are now also covered,'' a spokesman said.
"Snow is expected to affect higher roads and passes of central and southern New Zealand, with snow forecast to low levels across eastern and southern parts of the South Island.''
They heaviest snowfalls are expected in Fiordland, MetService said, where a heavy snow watch also remains active.
Snow falling above 300m should lower to 100m late this afternoon or evening before rising above 500m tomorrow morning.
"Periods of heavy snow are expected above these levels during this time and accumulations may approach warning amounts.''
Aucklanders were expected to see showers that will get more frequent and heavier, MetService said.
More strong westerly winds are on the cards too - particularly in exposed parts of the city.
A significant high of 15C is expected in the City of Sails and an overnight low of 9C. Just after 10am, the temperature outside felt like "three clothing layers'', the weather authority said.
Those in Hamilton will see similar conditions, save for some hail in the mix and possibly thundery conditions.
Despite the cold and gloomy conditions for many, some places are much luckier. Some regions will remain sunny and mild, according to WeatherWatch.
"By Friday, a warmer westerly returns,'' head analyst Phillip Duncan said.
"The weekend also has windy westerlies and a fast-moving cold front - classic front set up.
Snow is expected to quite low levels today and tonight and a few flakes could fall to sea level in the South Island, Niwa said.
A number of road snowfall warnings have been put in place and are valid until 10pm.
The areas affected are the Napier-Taupō (State Highway 5), Desert Rd, Lewis Pass, Arthur's Pass, Porters Pass, Haast Pass, Lindis Pass, Crown Range Rd, Milford Rd and Dunedin to Waitati Highway.
Motorists in the area are warned that showers may turn to snow and settle on roads above 600m.
Whangārei 17C high, 9C overnight low. Showers especially from this afternoon. Southwesterlies gusty this afternoon.
Auckland 15C, 9C. Showers more frequent and heaving late this morning. Westerlies strong in exposed parts.
Hamilton 14C, 6C. Showers with hail, possibly heavy and thundery from this afternoon.
Tauranga 15C, 7C. A few showers mainly from this afternoon. Gusty westerlies.
New Plymouth 14C, 7C. Showers, possibly heavy and thundery with hail this afternoon.
Napier 19C, 5C. Fine with high cloud increasing. Chance of shower from late afternoon.
Whanganui 15C, 7C. Showers with hail, possibly heavy and thundery from this afternoon.
Wellington 13C, 4C. Partly cloudy. A few showers from late afternoon. Northwesterlies strengthening then turning southwest this evening.
Nelson 14C, 4C. Fine at first, a few showers from this afternoon. Southwesterlies.
Christchurch 15C, -1C. A fine frosty start then showers developing from midday as northerly winds turn southwesterly - some possibly with hail.
Dunedin 10C, 3C. Showers developing morning with a southwest change, some possibly heavy with hail from this afternoon. Snow possibly lowering to 300m.