Compared with July, factory production volume fell by a seasonally adjusted 3.6% in August. Output was down 0.2% in the three months to August compared with the previous quarter.
A Reuters poll last week predicted a year-on-year increase of 8.3% in manufacturing output for August.
Kamilla Golda, an economist at ETM, said: "I suspect that it's reflecting how soft the manufacturing sector actually is.
"In the second quarter, you had temporary factors boosting manufacturing, such as the World Cup tournament.
"Without those temporary factors boosting manufacturing production, it highlights how soft domestic demand still is, and there are signals that external demand is also slowing, so manufacturing production could come under more pressure."
Factory production contributes about 15% to gross domestic product but remains fragile after last year's contraction.
The Purchasing Managers' Index, a key indicator of industrial activity ahead of official data, fell back into contraction territory last month, partly because of a strike in the automotive sector.
The recovery of manufacturing output has been hindered by demand remaining low in the aftermath of a recession last year, the country's first in nearly two decades.
Rand strength has also undermined the sector, hitting the exporting companies in particular.
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