As I understand it, Jetstar lease the A330's from Qantas but Qantas retains the freight capacity/availability. For this, Qantas pay a fixed lease fee to Jetstar for the freight capacity whether a single item of freight is carried or not and regardless of whether or not Jetstar is using the A330 on a route that ever even carries freight.
Qantas then charge Qantas freight only for the actual freight carried.
I was once told the lease fee paid by Qantas to Jetstar for this freight capacity was so generous that it could never be recouped no matter how much freight was carried.
It was even suggested that Jetstar could operate the A330 to Japan with not a single passenger and still make a profit due to the freight charge to Qantas. I find that just a little too hard to believe though not beyond possibility.