assuming we're allowed to travel by early 2022, we're considering a self drive Perth to Sydney, but veer down through Adelaide and Melbourne..... we're recently retired, and in no particular hurry, so ...

It's a long haul from Perth to Adelaide across the Nullabor with very few towns along the way (as Australians we've done it several times) so the suggestion to take the train is a good one. Hire a car ...