You can get flights with Virgin Blue right now for $69 each way, however most of those tend to be around the middle of the week.
I flew from Sydney to Melbourne just the other week for $100, and that flight was taken down to $85 a couple of days before I flew. The cheapest flights are usually the ones around the middle of the day, or early morning.
Virgin Blue are definately a better bet than Qantas though, although that's common knowledge really.