As a result of stumbling across this, I think I've finally found out how and when my great great grandfather, George Mawbey, arrived in the new province.
A name that sounds like his is on the manifest of the Nereus that sailed from Sydney and arrived in Adelaide on 27 January 1839.
The name given is 'Mr G Morley', but it is possible that it was written phonetically, not the correct spelling.
When Geo. Mawbey's wife, Ann, was planning to sail from Sydney to join him, she too was booked to travel on the Nereus.
But bad weather forced the vessel to turn back, and she subsequently sailed on the Abercrombie.
I'll see if a 'Morley' appears in any South Australian newspaper articles or directories at that time.
The link to this Pioneers and Settlers of South Australia website is StateLibSA-ShippingRecords.