The Royal South Australian Almanack 1839 does not list George Mawbey in its business directory, but does the following:
John Stuckey, baker
J Pitman, joiner and pianoforte tuner
John Reynell, landholder, 'Surreyville', 81, Rundle Street
Archibald Macdougal, printer
George Alston, accountant, 41, Rundle Street
Dingham and Harvey, merchants
Flaxman and Rowlands, merchants
John Forrester, victualer
Miss Nihill, ladies school
___ Williams, builder
E Baker, Adelaide Bazaar
John Crawford, builder
Clement Crispe, butcher
At the beginning of 1839 when this alamanack was compiled, it was not two years since the founding of the new colony, and there were already 620 houses and 5,000 inhabitants.
[Source: Royal South Australian Almanack 1839]

UPDATE 19-12-11
According the Royal South Australian Almanack 1840, John Reynell, was a director of the Joint Stock Cattle Company. It was established to 'supply colonists with the purest and most approved breeds of cattle, as well as butter and cheese at a moderate price'.
Mr John Stuckey, baker in Rundell Street, had been appointed one of their agents for butter.
Archibald Macdougall published the Southern Australian every Wednesday morning, price 1s.