In general, immigrants were hired on arrival in Sydney. See 1848 'IMMIGRANTS PER ROYAL SAXON.', The Sydney Morning Herald (NSW : 1842 - 1954), 20 July, p. 2 for a description of the process. The squatters with larger properties (such as Flood) would have employed agents for the task of securing new employees on annual contracts each time a ship arrived.
It would certainly have been possible to pre-arrange these matters in England, but the employer then ran the risk that his man would be one of those who died during the voyage.
Remember that at this time, the colony of New South Wales was making the difficult transition from a penal colony to a free settlement. The big landowners were accustomed to being assigned what amounted to slave labour.