"We're going to rescue them and we're just trying to reassure people to stay in their vehicles rather than get out and walk," he told NZPA tonight.
Emergency services would likely use a four wheeled-drive vehicle and snow plough to clear the road and rescue the motorists, he said.
The road had been closed and snow was still falling in the area, he said.
Snowfall appeared to be starting to ease in other parts of Wellington, Mr Coleman said.
Earlier, snow had settled in Wellington's northern suburbs, including Churton Park, Newlands, Karori, and there was flooding in Kilbirnie.
Police had received several reports of breakdowns and minor accidents, but no major accidents, he said.
Police advised motorists to stay off the roads in Wellington, particularly in the northern suburbs, where conditions were icy, Mr Coleman said.
The motorway was still drivable, but people had got into difficulty there too.
Anyone who became stuck in a vehicle not on the Rimutaka Hill Rd, would need to decide whether to stay in their car or venture out if they had warm clothing, he said.
Several state highways had been closed in the lower South Island because of snow and ice, police said.
Motorists should ensure they can complete a journey before starting it and drive to the conditions.
DUNEDIN
Dunedin police told NZPA there had been a few minor crashes and roads were getting icy and impassable.
It was probably not worthwhile driving in the hill suburbs, police said late this afternoon.
Anyone who did not have to drive should stay home.
It had been snowing on and off all day, not the worst snowfall the city had seen but it did not take much to make the roads dangerous.
"It's probably going to get worse as the night closes in and things start to freeze."
Senior Sergeant Anton Tenhove, of Southern police communications, said late this afternoon there had been reports of snow in Dunedin and other places in the South Island, but no major issues.
It had snowed in Christchurch for about half an hour, but become fine again, he told NZPA.
"We're expecting it to get worse overnight."
The MetService said a polar outbreak was bringing snow to southern and central New Zealand.
It forecast significant accumulations in Fiordland, south of Te Anau, Southland and Otago.
Snow would fall to low levels in Canterbury, parts of Marlborough, Wellington, Wairarapa, Horowhenua Kapiti Coast, Manawatu, Wanganui, Taihape and Taranaki - with significant amounts expected to accumulate by midnight tomorrow.
Heavy snow was likely to last until Tuesday in Canterbury, Marlborough and the Lower North Island, but then ease further south, the MetServive said.
This afternoon it tweeted that the temperature in Wellington dropped 8degC to 5degC in half an hour.
CHRISTCHURCH
Christchurch police urged people to check conditions before travelling during the next few days and drive to the conditions.
"We are asking the public to stay at home and delay all non-essential travel if they do not think they can travel safely in their local conditions," said Inspector Al Stewart, Canterbury road policing manager.
Invercargill police said there had been some snow flurries and hail, but no major issues.
HIGH / MEDIUM RISK AREAS
The MetService had issued road snowfall warnings for the Desert Rd and Rimutaka Hill Rd in the North Island, and Lewis Pass, Arthurs Pass, Lindis Pass, Porters Pass and Milford Rd in the South Island.
Snow was forecast to fall across the North and South Island to levels not seen for decades, Weather Watch forecaster Phillip Duncan said.
He said there was a high risk of snow settling in:
Christchurch, Ashburton, Dunedin, Wellington's higher suburbs, Upper Hutt, Palmerston North, Queenstown, Oamaru, Kaikoura, Gore, Balclutha, Alexandra, Waiouru, Ohakune, Stratford, Hawera, Taupo, Taihape, Masterton and Eketahuna.
There was a medium risk of snow flurries and possible settling snow in:
Invercargill, Timaru, Wellington's CBD, Lower Hutt, Wainuiomata, Kapiti Coast, Levin, Wanganui, Napier, Hastings, New Plymouth, Murapara, Hawera, Nelson region to sea level, Dannevirke, Carterton, Greytown, Rotorua's higher suburbs and Blenheim.
Snow hits Dunedin, arrives in Christchurch
For the first time in memory for many people snow is being forecast to sea level across a number of North Island and South Island centres, including three of our biggest cities.
Wellington's higher suburbs, Christchurch and Dunedin are considered to be high risk areas that should see snow settling until Tuesday morning.
Snow has been falling in Dunedin and has now arrived in Christchurch, as a massive low works its way up the country.
Parts of Dunedin have received heavy snow today, while Queenstown was also hit.
Flurries are now being reported in Christchurch.
Invercargill has had a few flurries but so far our southern most city has avoided the bulk of the snow, WeatherWatch.co.nz is reporting.
Hail showers are popping up in many places right across the country - as far north as Auckland.
WeatherWatch.co.nz is forecasting snow as far north as Northland tomorrow, down to just a few hundred metres, and is likely to be heavier than the July snow storm.
Temperatures for tomorrow and Tuesday are forecast to be the coldest of the year, with the possibility of Wellington receiving snow down to sea level.
A dusting of snow is also expected to fall in Napier, Taupo and even Rotorua.
Weather Watch senior analyst Philip Duncan said the low-pressure system was one of the biggest he's ever seen.
"This set-up is something that you see only a few times in a lifetime.
"The highest risk area will be Wellington. There is a risk of snow settling to low levels in Wellington."
Duncan said heavy snow may close the Desert Road and State Highway 2 at the Rimutaka ranges.
"Motorists may find the only way they can drive to Wellington is through Taranaki," he said.
The storm will travel north in waves, Duncan said.
The first will pass over the North Island tonight and the second, more-serious blast will pass over on Tuesday night. People in Waikato and Auckland would possibly see snow on the mountain ranges in their areas, Duncan said.
"This is Auckland's best shot of getting snow.
"If this storm had formed more to the west, I have no doubt that snowflakes would be falling over Auckland. Freezing level will be similar to July. We don't expect the levels to be any lower, but there will be probably more snow."
