Commsec dont provide an easy answer for this. Any order you put in goes directly to market. A sell order placed below current SP will sell immediately.
The only way you can do this is to pass a test and have conditional order trading set up on your account. They make you sit a online test to ensure you understand when and what you are doing lol... If you pass it provides you with the option to place falling /rising sells and more. This type of order will only be made to the market when certain criteria re SP and or volume are met (you set these). You are obviously charged a little more for this trade.
