Mezue ahịrịokwu na-esonụ: Na Septemba 1796, a họpụtara Johnston ka ọ bụrụ onye enyemaka nke Gọvanọ John Hunter, na 1800 natara ọkwa brevet ya dị ka onye isi. N'otu afọ ahụ Johnston nọ n'okpuru njide nke Lieutenant Governor William Paterson na ebubo nke "ịkwụ ụgwọ mmụọ nye onye ọrụ dịka akụkụ nke ụgwọ ya na nnupụisi nke iwu". Johnston ekweghị ka e kpee ya ikpe n'ụlọikpe ndị agha dị n'obodo ahụ, Hunter zigara ya England. N'ebe ahụ, ihe isi ike ndị dị n'iji ndị akaebe nọ n'Australia kpee ikpe mere ka a kwụsị ikpe ahụ, Johnston laghachikwara New South Wales na 1802. Na 1803 Johnston weghaara iwu nwa oge nke New South Wales Corps n'oge ọrịa Paterson, wee tinye aka na esemokwu dị n'etiti Eze na ndị agha. N'ọnwa Maachị nke afọ 1804, o ji mkpebi siri ike mee ihe mgbe ọ na-achịkwa ndị agha zigara megide ụfọdụ
ndị a mara ikpe na-enupụ isi na Castle Hill. Mgbe e zigara Paterson na Port Dalrymple, Johnston ghọrọ ọchịagha nke New South Wales Corps.