The filling of casual Senate vacancies was crucial to the constitutional crisis of 1975.
Without the breaking of the convention surrounding casual vacancies, the blocking of Supply by the Senate would not have been possible.
The convention was straight-forward: In the event of a senator’s death or resignation, the Constitution requires that a replacement be appointed by the Parliament of the State represented by the senator. In practice, the convention was to appoint a replacement from the same party, so as to maintain the balance of numbers in the Senate.
Following the elections of 1974, the composition of the Senate was:
- Liberal/Country Party: 30
- Australian Labor Party: 29
- Independent:1 (usually voted with the ALP)
On these figures, the Senate could not block the Budget, or other money bills, because the vote would be tied, and therefore defeated.