Prime Minister of Australia prime minister, otherwise William McMahon63. Robert Menzies was the oldest person to ever be prime minister, leaving office at 71 years old. The longest-serving Prime Minister was Sir Robert Menzies, who served in office twice: from 26 April 1939 to 28 August 1941, and again from 19 December 1949 to 26 January 1966. In total Robert Menzies spent 18 years, 5 months and 12 days in office. He served under the United Australia Party and the Liberal Party respectively. The shortest-serving Prime Minister was Frank Forde, who was appointed to the position on 6 July 1945 after the death of
Acting prime minister Minister, served as Acting Prime Minister for six days, after Norman Kirk died in office in 1974. Deputy Prime Minister Winston Peters was serving as Acting Prime Minister as of 21 June 2018 until 2 August 2018, while Jacinda Ardern took maternity leave. Prior to the establishment of the Deputy Prime Minister role, a senior Minister would be appointed to act as Prime Minister when, for example, the incumbent was travelling overseas. In the current era, when both the Prime Minister and Deputy Prime Minister are unavailable, the next highest ranking minister who is available will be asked to act