The main place in Australia where it would make sense to make high speed rail is the big Sydney-Melbourne route as it's been the world's 3rd or so busiest air route for many decades now. Currently it's about a 2 hour flight at a cost of a few hundred dollars with flights leaving multiple times per hour. By car this journey takes 7-8 hours, and these are the cities where you can do a lot without needing a car, so there's no incentive to take yours.

The other places where it might make sense are Sydney-Brisbane and maybe Adelaide-Melbourne since both are also 7-8 hour drives and about 2 hour flights. Last I travelled Adelaide-Melbourne the trip by train was 13 hours and by bus it was about the same because of the route and extra stops.

As another commenter has pointed out, in the smaller Australian cities you pretty much need a car to get around. So realistically the train journey is competing with the 2 hour plane ride rather than the 8 hour drive. Because you'll either drive and have a car at your destination or fly/train and need a hire car or taxis when you get there.

Don't get me wrong, I like trains and I wish Australia was less like Los Angeles and more like Europe, but it would take serious investment by the Government to make this happen and compete with the airlines.