well, if you think about it QANTASTIC, it would cost the Qantas Group more money to hire an internal for a once-off ten month contract than an external.
If they put a Jetstar f/a (or another internal staff member) onto this contract, they not only have to pay for the training/ uniform/admin of that contractor, but the Group also has to pay to replace the internal, with another employee. So they are in effect emplying two people (with all the associated costs). And paying the costs all for one employee that will be with them for only ten months.
If an internal goes to a full-time position within flying, they still pay the costs of replacing them, but it is a better investment as they are getting a full-time employee, who will presumably work for years to come.
If they give the contract to an external applicant, then they are only paying to employ that person, as it doesnt create a vacancy in another part of the group.
Just myn personal view of the situation !