Senators who represent states are elected for 6 year terms. Unlike Members of the House of Representatives, they are not usually all elected at the same time. Every 3 years the terms of half of the state senators expire, allowing for a complete rotation of senators every 6 years. This is called a half-senate election.
Senators representing the Australian Capital Territory and the Northern Territory have 3 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 2 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.