This stamp led me to an interesting page on Stamps of Victoria, so I thought I'd share both.

and
http://www.stampsofvictoria.com/english.phpIn a nutshell, mail bound for England would leave Melbourne by ship, and the ship would then call at Adelaide a few to several days later. A train connection between Melbourne and Adelaide allowed mail that missed the ship in Melbourne to still catch the ship in Adelaide. Procrastination must have been alive then as now, because it apparently filled a need and thrived.
Mine appears to be a Type 3-Out cancel. I presume "Out" signifies Melbourne to Adelaide.