Temperatures were beginning to fall over the deep south as a southwesterly front started to pick up this afternoon, he said.
Canterbury and Marlborough appear to be in the firing line with heavy snow forecast.
Snow could reach 1m deep across higher altitudes in Canterbury, with substantial falls of up to half a metre possible about the upper parts of the Canterbury plains.
Christchurch city may also see up to 20cm of snow by later tomorrow if sea level temperatures remained bitterly cold, he said.
Christchurch could see temperatures struggle to hit four or five degrees by lunchtime tomorrow, "with the chilly southerly they'll feel more like sub-zero''.
"It appeared this morning that rain and snowfall totals had eased a little but this afternoon it appears they're back and packing a punch.
"This is what can happen and can be a bit of a forecasters nightmare,'' said Mr Green.
"Snow levels have fluctuated and once again it's looking like a near sea level event with some big totals possible further inland.''
Snowfalls of this amount can be dangerous for drivers, stock and also households, as powerlines can struggle under the weight of the snow.
Forecasts for the rest of the country include wind and rain with severe wind gusts possible for the southwest corner of the North Island.
Overnight high winds from the southeast could hit Nelson, Motueka, Takaka and further south in Westport, said Mr Green.
Rosa Peacock from Orairi Gorge Station in Geraldine said stock had been moved off the high country.
"We've got our big annual bull sale tomorrow so we're more worried about buyers being able to get here than anything else to be honest,'' she said.
Canterbury police were advising drivers to assess local conditions before travelling.
"If conditions appear difficult, drivers should think about looking for alternative transport, or consider delaying any non-essential travel where possible,'' said Inspector Al Stewart, Canterbury road policing manager.
"Drivers need to be aware that while the snow will be a clearly visible hazard, there may be large areas of ice on the roads later that they will not be able to see - especially in rural and shaded areas.''
Canterbury Civil Defence took to its Facebook page to warn Cantabrians of the dangers.
"Please check road conditions before heading out, consider taking some emergency supplies if you do travel, esp if going near alpine passes. A good opportunity to also check household preparedness. Take care everyone.''
Christchurch City Council warned of a high tide in the Lower Avon River and Lower Heathcote River between 6pm and 8pm today.
The peak tide, combined with an incoming low pressure weather system, could result in surface flooding in certain areas.
Transport and greenspace spokesman Alan Beuzenberg said "the council will continue to monitor high tides expected in coming days and will take action to address any surface flooding issues if required.''
Meanwhile, freak tides could flood parts of the Auckland motorway network tonight and tomorrow.
The New Zealand Transport Agency said the areas at most risk were the northwestern motorway causeway between Pt Chevalier and Avondale and the northern motorway southbound from Esmonde Rd to the Exmouth Rd footbridge.
Critical times will be high tides at 8pm tonight and 8am and 9pm tomorrow.
- By Brendan Manning of APNZ
