If Qantas mainline want to replace 737's with a320's then I suspect it will be crewed within mainline. There are approx 600 737 drivers. If the firm had thoughts of crewing externally or from other parts of the group,why would they be hiring second officers thereby increasing the number of mainline pilots? All pilots within the Qantas group are flying max hours as it is.
If it is their move to have another pilot group fly the 737 replacement, then you would have thought the current short haul pilot group would via seniority just slide back into long haul with the impending 787 arrivals. Otherwise we will have another major surplus of pilots which would lead to another round of rin, vr or even cr. However this does not seem to be the case nor do I suspect it will be.
No doubt though what will probably happen is when the new aircraft type is announced, the company will put pressure on the union and negotiations will take place to determine the proposed conditions. The current short haul contract is efficient enough as it but that is for another discussion.