But the automaker said on Monday it had sufficient inventory for both domestic and exports customers for the medium term.
Over 200,000 members of the National Union of Metalworkers of South Africa (NUMSA) stopped work last week demanding salary hikes of up to 15 percent, in a strike that will further hurt the ailing economy.
NUMSA has rejected an updated 10 percent offer from employers' group Steel and Engineering Industry Federation of Southern Africa.
"The strike in the metal and engineering sector has impacted upon supply of components to our production line, resulting in our line not being operational since July 3," GM spokeswoman Denise Van Huyssteen said. "To date we have lost three days of production."
Mercedes Benz said there had been no impact so far at its local operations, while Toyota said it was still at "full production".
The NUMSA strike comes hot on the heels of another five-month work stoppage by miners in the platinum sector that choked output in the key industry.
The union wants any wage hike agreement to apply for a year only but companies want a three-year deal.