For domestic, just ask at check-in (or at the Qantas Club if you have access). As long as you fit the requirements, and a seat is available they should give it to you.

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.