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
Prime minister-designate for a party leader who is leading in the polls and/or has a meaningful chance of winning, or even more generally at any time between elections in reference to any opposition party leader (regardless of his or her party's perceived electoral prospects) and even for future leadership contenders within the current governing party. Under the broader definition, many prime ministers-in-waiting never actually become prime minister. In some countries the role is specifically covered by legislation, in others convention applies before the chosen leader is sworn in. The Australian Electoral Commission, the government authority responsible for the conduct of elections in