Jamison had been appointed a magistrate following his return to Sydney. In 1805, he court-martialled two assistant surgeons for failing to attend women giving birth but this punitive course of action was later over-ruled by the British War Office on a technicality. Also in 1805, he received a grant of land on the Nepean River, west of Sydney, where he raised livestock and grew crops using assigned convict labour. Later, he acquired another of agricultural land at South Creek and on the Georges River. He also erected a house in central Sydney (in what is now Jamison Street).