Marcus said the policy decisions the US Federal Reserve was likely to take on quantitative easing posed a "real risk" of sudden large outflows of capital from SA.
Marcus and her adviser Brian Kahn were speaking at a parliamentary briefing to the standing committee on finance on Wednesday.
Kahn said there was an "urgent need" for South Africa to increase its exports if it wanted to address its balance of payments problem.
He noted that the bank's annual economic report said SA was vulnerable to capital reversals because of its "persistent current account and fiscal deficits", which needed stable financing.
The country was exporting less in terms of volume, and its export prices were moderating.
"Overall, in value terms, it is not a very positive story," Kahn reportedly said.
South Africa's exports fell by R5.8bn or 7.6% in August which contributed to the increase in the country's R19bn trade deficit, the paper reported.
In July, the deficit was at R13.4bn.