Unfortunately, this service - the 'Overland" - is a service which GSR is obligated to provide, but doesn't really want to. It is a hangover from when they bought the passenger operations of Australian National.
There is so much that they could do with this service, and it would certainly be within their means to do so. But they won't, they want to see the service die.
A pity, some of my best train trips have been from MEL to ADL and return.
Dave