Definitions for "Casual vacancy"
A vacancy in the Legislative Assembly caused by the retirement, death or resignation of a Member. Where a casual vacancy occurs in the ACT it is filled by recounting the votes received by that Member. (see by-election)
If a Senator retires or dies the casual vacancy is filled by a person chosen by the joint sitting of the Houses of Parliament of that State until the expiration of the term. The person chosen must be of the same political party as the predecessor.
a vacancy that arises in between elections or where the number of representatives elected at the first or subsequent elections is less than the total maximum number of elected representatives but more than half that number