Ann: Market Update - Progress Report, page-100

If you read my previous posts, it's all explained. Briefly, there are people, day traders, who have very high speed connections to the ASX. Much, much faster than the local brokers - costs more obviously.
This allows them to submit a simultaneous sell/buy transaction which is executed in real time. There is only one person involved - the shares never change hands, but the transaction appears in the course of sales. And this can be confusing because we don't know which transactions are normal sell/buy between 2 unrelated parties, or whether the shares are just going round in a circle.
As well, these people can execute minor transactions - single figure numbers that you and I can't do, and still move the SP up or down.
None of these transactions ever show up in the depth, only in course of sales. It's always interesting to check out depth, and compare to course of sales. Often you'll find executed transactions that have never appeared in the market depth. It's annoying, but legal.