Perth doesn't count as a major city? (% of that 30% live within its metro area. This isn't to say the 7-city grid is a bad idea, but your statement is inaccurate.
Perth (and Darwin) are essentally islands of their own.
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.
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).
