Let me see if I remember the history...
Gough Whitlam becomes the Labour Prime Minister of Australia in December 1972
In 1973 Gough Whitlam selects John Kerr to be Governor General.
In 1974 Whitlam dissolves both houses of Parliament, and in the subsequent election wins a majority of the House but not the Senate.
In 1975 Australian state governments replace Labour vacancies in the Senate with non-Labour Senators.
In October 1975 the government fails to pass its budget.
In November 1975 the Governor General dismisses Whitlam and calls for new elections.
In December 1975 Whitlam and the Labour Party lose the election, and John Fraser becomes Prime Minister.
Looks to me like the Whitlam government was overthrown by Australia's voters in December 1975, when they refused to overrule the Labour Governor General's dismissal of Whitlam and returned Fraser's party with clear majorities.
"Coup d'etat" means something very different from this particular political episode.
Brad DeLong