background, CAMPBELL, Sydney
https://campbellsstores.com.au/history/ so from earliest British settlement of Sydney...
Also, Lachlan Macquarie, 'Father of Australia' (its on his headstone in Scotland), Governor of NSW post the first coup d'etat ... Rum Rebellion etc, deposing Bligh .... 26 January 1808 etc ...
Macquarie's wife, Elizabeth, her family name was CAMPBELL ...
http://adb.anu.edu.au/biography/macquarie-elizabeth-henrietta-2418 But we need to be looking at the period mid to late 1840s, for Thomas WALKER, so convictism ceased to NSW by 1840, he got his CF in 1843, and may well have been known by any name he chose to become known by. His marriage had quite within both NSW and English law, been effectively terminated back in 1828 with his sentence 'beyond the seas' for 7 or more years (he got 14, due to former convictions) ... so he had been free to marry from as early as mid 1829 (with the governor's consent while under the bond of that sentence, but completely free from 1843 issue of CF).
Clearly he was well trained for farming. But by 1843 he was no longer a young man ... so if he acquired his own land to farm, he would have needed to engage labourers to help. Remember too that in the mid to late 1830s, England had set up South Australia as a separate entity ... not as a penal colony ... so it had set up a private company to 'sell' land ... (NSW had been 'granting land', pastoral leasing etc, - differences are well documented) ... so the 'price' of land needed to return profit to the private company ... so NSW governor had to increase his 'upset' price and the 'quit rent' price etc ... so land became expensive ... so an economic depression befell NSW in the early to mid 1840s ... so .... where would a single chap with a CF go to start over again - in the years before huge populations flocked to 'Australia' with gold fever - commencing mid 1851 ?
I think of New Zealand, or 'Australia Felix' (ie that part of then NSW that became Victoria), or Cape Town (South Africa), or Peru, Chile or California or perhaps South Australia ... or he stayed in and around Queanbeyan or the Monaro -
Hope those suggestions are helpful.
JM (yes, Neale is right about the Campbells - umm.... yes, there's a rogue Campbell on one side branch of my own tree !)