Perth-Adelaide (nearest major metro) is 2,131km air mles, over the Great Austrailian Bight (ocean). Land route is 2,695 km (1,675 mi).
Darwin-Brisbane is even further: 2,848 km by air, 3,425 km by ground (2,128 mi).
By 300 kph HSR, those are 7+ hour trips, vs about 3 by air. And those are nearest-nighbour trips.
There is effectively no population between either location -- unless somehow induced (a possibility), a high-speed rail service would have tpo rely on endpoint-traffic only. Climate, economy, and ag productivity (low) make development unlikely.
https://www.distancecalculator.net/from-perth-to-adelaide
https://www.distancecalculator.net/from-brisbane-to-darwin
By contrast, the southeastern zone of Adelaide-Melbourne-Canberra-Sydny-Brisbane, is compact with five major metros over 1,601 km (air), though the ground route is still 2,005 km (1,246 mi).
https://www.distancecalculator.net/from-adelaide-to-brisbane
And still, the only HSR corridor that's been seriously considered is SydneyMelbourne. Quite validly.
Australia is almost wholly empty space.
https://www.openstreetmap.org/#map=5/-24.921/134.272
Populations:
Perth: 2m
Adelaide: 1.7m
Melbourne: 5m
Canberra: 0.4m
Sydney: 5.2m
Brisbane: 3.6m (metro area)
Darwin: 0.1m
Giving mobility to those people would unlock new use cases of its own, but the GP post is actually accurate to commercial reality.