This question comes up on HC quite regularly. The answer is explained on the ASX site, see link below.
It is a good idea to understand this process, as it can often be used to your benefit, should you wish to put yourself at the top of the queue but get the same price as you would get if you were at the bottom of the queue, subject to minor risks.
Go to http://www.asx.com.au/markets/l4/OpenClose_AM4.shtm to find out how the single-price auction works.
