Hi there,
It is entirely possible that the registration of his death failed to reach the BDM authorities, particularly if it occurred during the Gold Rushes of the 1860s. I know that at least in NSW, the deputy registrars were meant to send quarterly summaries to the Sydney HO, and that these were usually despatched by Royal Mail. And the Royal Mail went by Coach along with the Gold ! and errr..... not all the coaches managed to get to their destinations without being 'bailed up'. Sometimes even the Royal Mail was removed by the bushrangers !
Alternatively, there are numbers of death registrations where the surname was not known to the informant (usually a police officer or a hospital attendant), usually as a result of a sudden unexplained death.
I am not sure about Victoria, but I do know that in NSW, up until WWI, it was entirely possible for a Police Magistrate, or a Coroner to issue an order for a burial, often before any inquest was held. Then they simply overlooked registering the death, (the regulations did not specifically direct either to register the death) or request the funeral director to register it.
Cheers, JM