on the confirm page, there will be code similar to:
$AuthNet_optional[0][$nextIndex] = "x_currency_code";
$AuthNet_optional[1][$nextIndex] = "USD";
that will need to be changed to use your currency:
$AuthNet_optional[0][$nextIndex] = "x_currency_code";
$AuthNet_optional[1][$nextIndex] = "ZAR";