Prime Minister of Australia
The Prime Minister of Australia is the head of government of the Commonwealth of Australia, holding office on commission from the Governor-General. The office of Prime Minister is, in practice, the most powerful political office in Australia. Despite being at the apex of executive government in the country, the Prime Minister is not mentioned in the Constitution of Australia and exists through an unwritten constitutional convention.
Barring exceptional circumstances, the Prime Minister is always the leader of the political party or coalition with majority support in the House of Representatives. The only case where a Senator was appointed Prime Minister was that of John Gorton, who subsequently resigned his Senate position and was elected as a member of the House of Representatives (Senator George Pearce was Acting Prime Minister for seven months in 1916 while Billy Hughes was overseas).