The bus involved was ferrying staff from the Venetia mine, one of the biggest diamond mines in the country, a transport official in Limpopo province, in the north of the country said.
"The bus came into collision with a lorry," Vongani Chauke told AFP.
Chauke said the accident happened at around 16:00 GMT, some 25 kilometers (15 miles) from the mine, at the village of Musian on the border with Zimbabwe.
The cause of the accident has not been established and at the time of filing this report, it is unclear if others survived the accident.
South Africa has one of the most developed road networks on the continent, but also has one of the worst road safety records.
The Venetia mine, which lies near the borders with Botswana and Zimbabwe, has been run by the De Beers group for more than 30 years.
It accounts for more than 40% of the country's annual diamond production, employing more than 4,300 staff including many local people.