It's presumed that a Melbourne-Sydney high-speed railway link would mostly follow the corridor of the Hume Highway and the existing, low-speed, railway link, going through Albury; there are also proposals for a high-speed railway link between Sydney and Canberra (on which a journey would take only 50 minutes), and as sych, it may make sense to combine the two proposals and have the link go through Canberra and on to Albury and Melbourne.
The socioogeographic consequences of such high-speed rail links in Australia would be interesting; were all these lines constructed, Newcastle, Sydney and Canberra would become one mutually commutable conurbation; one could see people commuting to Sydney from Newcastle, politicians and public servants commuting to Canberra from their Sydney residences (and there's no reason why powerful politicians shouldn't use high-speed rail; the service will surely have first-class compartments of the sorts British and European politicians use). Meanwhile, further south, Seymour could become a new patch of Melbourne's commuter belt.
A Melbourne-Sydney high-speed rail link is said to cost about A$40bn, and would cut travel times down from 11 hours to some 3 or 4. Australia, its economy buoyed by demand for resources, is better placed economically to commit to such projects than, for example, the US or UK (who are pushing ahead with their own high-speed rail projects), though there remains a lot of inertia. As such, I'd be pleasantly surprised if Australia gets high-speed rail before, say, Sudan.