The N3 at Van Reenen's Pass between Harrismith and the Tugela toll plaza was closed for a second time on Tuesday because of snow, the N3 Toll Concession Limited said.
Spokeswoman Andy Visser said heavy snowfall in the early afternoon forced authorities to close the road again.
It was briefly reopened just before noon, after being closed in the morning due to the treacherous driving conditions associated with the winter flurry.
Visser said it had snowed along the N3 from Harrismith to Hilton, outside Pietermaritzburg.
Arrive Alive spokesman Tshepo Machaea said a number of roads in the Eastern Cape had been closed because of significant snowfall.
These included the R61 between Graaff-Reinet and Cradock, the N9 between Graaff-Reinet and Middelburg, the R58 in the region of Barkly East, and the R68 between Barkly East and Lady Grey.
Roads around Matatiele and Kokstad were also affected by heavy snowfalls, Machaea said. He appealed to motorists to respect the rules of the road and to exercise extreme caution.
Airports Company SA spokeswoman Unathi Batyashe-Fillis said there had been snow at OR Tambo International Airport, but that it had not been heavy enough to affect flights by midday.
"There is currently no word that flights have been reduced, but each airline makes their own call on this."
In the Free State, the Golden Gate Highlands National Park, operated by SA National Parks, had its heaviest snowfalls in six years. Hotel manager Gerrit Potgieter described the snow as "really thick" and said more was falling.
On social networks, South Africans posted photographs and text updates of snow around the country. In Johannesburg's CBD, people flocked onto the streets to see the snowflakes whirling between the tall buildings.
Earlier, Lynne Minnaar, owner of the Groenvlei Farm Guest House in Graaff-Reinet, said her lawn was dusted with snow on Tuesday morning.
She said she had received more bookings than usual as a result of the snow, and was hoping the white blanket would last until the weekend for the benefit of her guests.
Charlotte Meyer, the owner of the 26 on Chamberlain guest house in Queenstown, said snow had fallen there on Monday, and was still visible on the nearby Madeira Mountain.
She said snow fell in the area last year, but that prior to this there had been no snow for 41 years.
Residents of Vanderbijlpark, in southern Gauteng, awoke to discover a sprinkling of snow. It generally snowed in the area once every five or six years, said Pomegranate Bed and Breakfast owner Laetitia van den Heever, who described the scene as "just lovely".
"[The snow] has settled lightly on the plants. It's beautiful, it looks like Christmas."
Van den Heever said by 8am it was "snowing again in earnest".
SA Weather Service forecaster Jan Vermeulen said there were heavy clouds over Vereeniging on Tuesday morning, and a chance of light snow had been predicted over southern Gauteng.
There had also been reports of snowfalls in Bethlehem and Clarens in the Free State.
Parts of the Western Cape also experienced snowfall, and heavy falls were expected along the south-eastern coast.
Vermeulen said there would be a brief reprieve from the freezing conditions from Wednesday, but on Friday another cold front would hit the country from the south west, bringing a fresh wave of icy weather.