[6] He was Deputy Permanent Representative of Australia to the United Nations in New York 1984–1987; Australian Ambassador to Mexico, the Central American Republics and Cuba 1987–1989; Australian High Commissioner in Papua New Guinea 1993–1995 and in Malaysia 1996–1997; and Deputy Secretary of the Department of Foreign Affairs and Trade 1997–1998.

[3] In 2001, he also held the concurrent appointment of secretary of the short-lived Department of Reconciliation and Aboriginal and Torres Strait Islander Affairs.

[10] While in Jakarta, Farmer assisted in negotiations to return a group of 78 Sri Lankan asylum seekers to Indonesia after they were apprehended by the Australian Customs vessel Oceanic Viking.

[12][13] In retirement, he headed an independent review of Australia's live animal export trade,[14] tasked in June 2011 to examine the facilities, treatment and handling of livestock in Indonesia.

Farmer was awarded the Centenary Medal in 2003 and, in June 2005, was appointed an Officer of the Order of Australia for service to the community through contributions to Australia's international relations and to major public policy development including domestic security, border systems, immigration, multicultural affairs and Indigenous service delivery.