Three of the biggest South African banks - Standard, Absa and Investec - and 14 foreign-owned banks have been referred to the Competition Tribunal accused of price-fixing.
They are accused by the Competition Commission of manipulating the price of the rand with fake buy and sell positions to influence perceptions of the supply of, and demand for, the currency.
They are also accused of using, since 2007, internet chat rooms to co-ordinate sales of rands or the cutting-off of sales for a time to manipulate prices.
Banks found guilty of price fixing by the tribunal would be liable for a penalty of 10% of annual turnover.
"This is a big deal," said market commentator and trader Simon Brown.
"The Competition Commission is good at what it does and is not often wrong [about price-fixing] on everything from bread to bikes."
He said the value of the rand affected the price of everyday essentials, such as fuel, and so accusations of currency manipulation have implications for everyone.
The share prices of the three local banks barely moved after the announcement.
Brown said this might be because it was known that an investigation would take a long time.
"I am expecting a Twitter storm criticising the banks; they are under huge pressure in South Africa."
Most foreign banks had local assets that could be attached if they were fined.
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