Ann: Appendix 3B, page-5

If you want to cut your losses on a stock, don't believe in it and have made lots of money elsewhere and are going to end up with a tax bill it makes sense to get rid of the poor performing stock I in the same year if you haven't carried any other losses from other sales forward to offset the extra tax you'll have to pay. 
This happens a lot around June each year - Tax selling but the ATO cracks down on people doing this who buy straight back into a stock in the new financial year!