If no seats are showing they have already been taken! They are generally released one year out (as soon as the flight is available for sale). Many Qantas frequent flyers will stay awake into the early hours of the morning to snap up the seats as soon as they are available. Other airlines then have access to any seat seats left about a month later (roughly 11 months out).
More seats may be released during the year, but not something you can count on.
Options? Keep checking. Or look for partner awards on either Emirates or Cathay pacific. If using Cathay pacific, it help to look for SYD-HKG, and then HKG-BKK as a multi city search. Cathay often has much better availability, with more seats being released closer to departure time (in the 2+ weeks before departure).
If you are a Qantas platinum or above, you can call Qantas and ask them to release an award seat for you. They will try and help depending how full the flight is.