Hi leahm, MEL-PER is domestic and AFAIK, it's not generally possible to select a specific/particular seat on Qantas domestic flights.
QP and above can request forward/rear seating; all can request aisle/window.
The only normal way of selecting seats is either via OCLI, QuickCheck or asking the agent at a check in desk.
Now, if it were an international flight on Qantas, that's a different story ...