Petrie devoted much time to community welfare. For years he served on the management committee of the Brisbane Hospital and was chairman after 1885. He was also a member of the Board for Administering Outdoor Relief and the Central Board of Health. Appointed to the New South Wales Commission of the Peace in 1859, he remained a member of the Brisbane bench until 1892. He gave long service on the Brisbane Licensing Board and was often returning officer for the parliamentary elections. A trustee of the Brisbane general cemetery and of Bowen Park and a ranger for protecting native birds on the Enoggera Water Reserve, he was a director of several building societies and of the Queensland Steam Navigation Co. Elected to the North Brisbane School of Arts Committee in 1864 and 1866, he was also an enthusiastic member of the first Masonic lodge in Queensland.