IMHO We have massive distances and are missing the population density to make such a system viable. Japan has a distance of 2,400km of high speed rail line with a population of 127 million. Australia would require around 1,800km of high speed rail line to service the golden triangle. That would mean it's useful for as a guess 14 million people (populations of MEL + CBR + SYD + BNE + regional cities and town along the way).
Ok so not every one of those 127 million people in Japan has completely easy access to the trains, but lets assume at least 50% (and this number is likely to be much much higher) of the country does, it means that for every km of track in Japan there is 26,400 people who has easy access. In Australia, that ratio goes to only 8000 people per KM. So it means that at best, we have only a third of the population to support such a system.
That is not to say that such a system wouldn't be really really cool, but just been really really cool does not equate a good financial investment. Before I am convinced that a high speed rail system would be viable, I would like to see someone point to a country of similar size and population density (I'm happy to limit it to the golden triangle) whom have managed to make such a system run both profitably and well, because every example I've been told so far has talked about joining two cities together over relatively small distances and each city has roughly the total population located inside it.