C series would be a perfect replacement of the b717 and/or the b737-400 that mainline is operating. Bean counters would love to give it to the group that costs them the least. There are advantages of keeping them in house within the Qantas group (more controls of safety and standards), and there are advantages of outsourcing to Cobham.
There are previous orders of the c series from "undisclosed" customers, so this one isn't the only one that may potentially be an Aussie operator. But depends on what you are smoking/drinking on the day, some believe the excess a320s that Qantas/jetstar has ordered for their failed Singapore premium strategy, or the yet to be finalized jetstar hong kong.