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