Ive noticed similar patterns on long haul services.
Some read inventory seems to be release about a week ahead of time, even on LON-SYD. Oddly I've not seen much on SIN-SYD, but plenty on SYD-SIN and SIN-LHR. So if you are prepared to be flexible or book last minute, then things can work out well.
The other option, if you are Cash or Avios rich is to book something you can live with and then cancel if your preferred routing/destination comes up.