This is correct, if you hop just over the border you will get maximum 7 days to leave the country.
Overstays of less than 30 days end in ban for 1 year and with more than 30 days day will make you aan unwanted person for 5 years. Even if you applied for visa extension at home affairs. Since 26th MaiMay 2014 you can notcannot apply for an extension for vistors visavisitor visas in South Africa.