New Zealand Transport Agency regional network manager Murray Clarke said snow ploughs and grit trucks had been working hard to keep traffic moving on the motorway.
Traffic had been reduced to a single lane, resulting in long queues, as north and southbound traffic waited for turns to traverse the highway.
"It's been a bit of challenge up on the motorway."
A hail storm just before 8pm made roads very slippery, and continual snow flurries were hampering efforts to clear the road.
State Highway 87 between Outram and Middlemarch was also affected by snow.
Both roads were closed to heavy and towing vehicles, although once the queue of cars had thinned out, it was possible some trucks would be able to get through, he said.
Acting senior sergeant Simon Mapp, of Dunedin, said police had not been called to any incidents on the northern motorway, but there had been reports of trucks slipping and sliding.
He urged people not to travel on the motorway unless absolutely necessary.
Reilly's Towage and Salvage manager Brian Lilley said traffic on the northern motorway had been at a standstill at 5.45pm, but was later moving. Three Mile Hill Rd was closed, he said.
Motorists were being advised to watch for ice and grit in shaded areas and bridge decks, and to carry chains when travelling on alpine passes.
MetService forecaster Derek Holland said the poor weather was expected to clear by tomorrow afternoon, but could leave icy conditions in its wake for a day or two.
Snow fell in parts of Gore and West Otago but the closest most of South Otago came to snow was in the form of flurries late morning.
Cold winds and regular hail showers struck Balclutha during the day.
