May I please make some comments....
1. Mail held at Geelong at that time could well be mail from 'overseas' simply addressed to Mr XYZ, at that seaport, so it is possible mail held there could be from UK family and/or UK employer, and be sent there simply because that was the sea port that the sender was aware of. Mail being at that location does NOT, of itself mean that the addressee provided that address, or expected to collect mail from that location. In fact, perhaps it suggests the sender was striving to anticipate a location for the addressee, rather than acting on exact information provided by the addressee.
2. Mail listed as unclaimed in the Gazettes and newspapers was usually ONLY listed once or twice, and just because it was not listed in later months, that did not indicate it had been collected. It simply indicated that the Post Master General had determined a course of action to limit the accumulating mail being held. ...... Dead Letter Office/Return to Sender/irregular source of fuel for PO fire places etc.
3. Assayers offices were required by the various Colonial governments to be established basically at the gold fields, and of course Ballarat had assayers offices. It was the gold-dust/nuggets (so unprocessed) that was sent from those offices under government escort in strong boxes on royal mail coaches to the capital cities, and it was that gold that became the targets of bush rangers.
Cheers, JM
I agree it is only a theory. To be modified or discarded as more facts come along.
But to have a letter waiting for him at Geelong, he must have told someone to address it there. Which means that must have been the nearest office to where he was living at the time. I hadn't spotted it was a different letter to the on line reference, so we don't know how quickly he picked it up from Geelong, if at all.
But if there are records of gold sales available from towns near any places he was known to be, then it might track him. Just thinking of any other records that might be kept which might help locate him in a particular place at a particular time.
It's always hard to track people who are honest, hardworking, never commit any offence which would get them onto a court record or do anything oustandingly bad or good that gets them in the newspapers.