Senators who represent states are elected for six year terms. Unlike Members of the House of Representatives, they are not usually all elected at the same time. Every three years the terms of half of the state senators expire, allowing for a complete rotation of senators every six years. This is called a half-senate election.
Senators representing Territories have three year terms which are concurrent with House of Representatives terms.
So what happens after a double dissolution, given that all senators' terms start on the same day? How do we get back to the half-Senate election pattern?
Luckily the drafters of the Australian Constitution realised this problem and included in the Constitution a way to avoid all senators standing for election at the same time. Section 13 states that after a double dissolution, state senators are to be divided into two groups; the first group is to have a three-year-term, while the second group is to have a six-year term.
But how does the Senate work out which state senators will have a half term and which senators will have a full term?
In the past, the state senators with the highest number of votes in each state have been granted a six-year term. Three-year terms were granted to the state senators with the lowest number of votes in each state. This re-established the normal pattern of Senate elections.