If the place of a senator becomes vacant before expiration of a term, for example, by death or resignation, the Constitution provides (s. 15) that the vacancy shall be filled by the state Parliament, both houses, in all cases except Queensland (which has a unicameral Parliament), sitting and voting together. Should the state Parliament not be in session, “the Governor of the State, with the advice of the Executive Council thereof, may appoint a person to hold the place until the expiration of fourteen days from the beginning of the next session of the Parliament of the State or the expiration of the term, whichever first happens”.
As a result of an amendment to the Constitution passed in 1977, where a vacancy is left by a senator who, at the time of election, was publicly recognised by a particular political party as being an endorsed candidate of that party and publicly represented to be such a candidate, “a person chosen or appointed under this section  in consequence of that vacancy, or in consequence of that vacancy and a subsequent vacancy or vacancies, shall, unless there is no member of that party available to be chosen or appointed, be a member of that party”.
The purpose of this provision is to maintain the integrity of the proportional method of voting introduced in 1948 so far as the filling of casual vacancies is concerned. From the inception of this system of voting until 1975 such vacancies as arose were, by convention, filled by people of the same party affiliation. In 1975, however, two casual vacancies, both involving senators from the Australian Labor Party, one in New South Wales (arising from the resignation of Senator L.K. Murphy), one in Queensland (arising from the death of Senator B.R. Milliner), were filled by non-ALP candidates.
The current section 15 of the Constitution has not fully resolved the problem of filling casual vacancies caused by the death, resignation or disqualification of a senator in a manner which preserves the representational strength deriving from the proportional method of election. Further analysis of this aspect is contained in Chapter 4, Elections.
The decision of the electors in adopting a replacement section 15 of the Constitution in 1977 for filling casual vacancies is a clear demonstration of public support for the proportional method of composing the Senate embodied in the 1948 legislation. Other examples of support for this method may be found in its adoption for electing Legislative Councils in New South Wales in 1978, South Australia in 1975, Western Australia in 1989 and Victoria in 2003.
In order to preserve equality of state representation in the Senate, and to maintain proper representation of electoral opinion, the Senate has taken a close interest in prompt filling of casual vacancies when they arise. This matter is covered more fully in Chapter 4.
Chapter 4 also includes information about filling casual vacancies arising in the representation of the Australian Capital Territory and the Northern Territory.