I would hope as Jaded Boiler says, that it is to supplement the QF services. I would think this service would be quite profitable for QF as J class is very full 80-90% and ecy is also very full. Also there is a large ammount of cargo carried from SIN-CNS with connex to QF flights to NRT.
I don't see how Jetstar could make more profit than QF on this service as the ammounts and type of cargo carried will not fit on an A320 and QF has a high pax load. Even if the pax component of the flight is not yielding enough (is this the Pilots, FA or Ground staffs fault? No) , the cargo component should more than make up for it.
The only way Jetstar could do this would be with an A330. This may be done, who knows what they are thinking. Soon QF will only be doing circuits of SYD as they keep losing destinations to AO and Jetstar.