International is a bit more tricky. Only Plat's (and Chairman's Lounge) can pre-book an exit row with Qantas. However, when you check in, if there are seats available and you fit the requirements needed then they should be able to get you one.
Often exit rows are available at checkin. Domestically - because people online check-in/Machine check-in and don't bother asking, and Internationally because those who are pre-assigned exit rows (Plat/ CL) have a high chance of getting their upgrades using points granted. So they move, leaving the seat free.
Thanks for your advice...its definatly worth a shot! Ive never had an exit row for a longhaul flight, but I imagine it makes quite alot of difference to comfort!
I would check to see whether the exit rows are taken before I leave for the airport, but seat maps on sites like seatexpert & check my trip seem to be disabled about a week before the flight which is frustrating!!