I heard from a reliable source that the A350 delay in AUS was a legacy of when CASA put Thai on notice for lapse in safety and maintenance back in 2015. Thai need to jump through a lot more hoops at the moment by validating the maintenance and safety plan for their new aircraft.
The message that CASA hadn't seen a A350 come up for approval before was misleading. It's because casa hadn't seen an A350 by TG before, so they need to treat it as a new plane (remember, each airline has slightly different mods, and maintenance plans of the plane.. it's not just the logo that's different)