As this very helpful Point Hacks article states, Qantas opens up its reward seats up to 353 days before the flight. So you can expect reward seats for August 2019 to start becoming available progressively after about August 12 this year.
If you’re after a Premium Economy or Business Class seat, it will very much help if you have Gold or Platinum status with Qantas, because it’s very likely that the reward seats will become available to the Qantas Frequent Flyers with higher Status first.
If the past is any indication, there will be sporadic availability of seats — but not on every flight. If you’re willing to be flexible with dates, and especially if you’re willing to fly Economy, you’ll have a pretty good chance.
If you’re willing to forego the Dreamliner and fly Emirates, Cathay or another Oneworld airline (via somewhere in Asia), your chances will increase even more.
All the best!