Senators are elected by a preferential voting system known as proportional representation.
Candidates for the Senate stand for a state or territory. It is a Constitutional requirement that each state be equally represented regardless of its population.
There are a total of 76 Senators: 12 for each state and two for each territory. Senators for each state are elected for a six year term. Senators for each territory are elected for a term equivalent to the duration of the House of Representatives. When a House of Representatives and half Senate election are held at the same time, 40 Senate vacancies are contested.
When a Double Dissolution is declared, as there was in 1987, all 76 Senate positions are made vacant.
The order of the candidates on the ballot paper is determined by a random draw conducted in the office of the Australian Electoral Officer for that state or territory, following the public declaration of nominations. Senate ballot papers are white in colour.
The ballot paper is divided into two sections. Voters have a choice of two methods when voting for Senators; 'above the line' and 'below the line'.
A voter may vote for a political party or group by putting the number '1' in one box only above the black line. The rest of the ballot paper must be left blank.
By casting a vote this way, voters are allowing the order of their preference to be determined by the party or group they are voting for.
A Senate group may lodge a written statement setting out a preference order of all candidates in the election. This is referred to as a group voting ticket. Instead of a voter numbering all of the boxes 'below the line', the AEC will automatically allocate preferences in the predetermined order outlined by the particular party or group.
Ungrouped candidates do not lodge a group voting ticket and so do not have a box above the line on the Senate ballot paper.
A group voting ticket looks similar to a completed Senate ballot paper. A booklet with all the group voting tickets for the voter's particular state can be viewed at a local polling place. The group voting ticket clearly shows the order in which the party or group will allocate a voter's preferences, or votes. Each party or group can register up to three group voting tickets.
A voter can choose to fill in every box below the line in the order of their preference by putting the number '1' in the box of the candidate they want as their first choice, number '2' in the box of the candidate they want as their second choice, and so on until all the boxes have been numbered. The top part of the ballot paper must be left blank.
If a voter chooses to vote below the line, they must number every box below the line for their vote to count.
An informal ballot paper is one that has been incorrectly completed or not filled in at all. Informal votes are not counted towards any candidate but are set aside.
* This means, for example, if there are twenty candidates, a ballot paper would be informal if it did not have either the numbers 1 to 18 (90% of 20) marked without repetitions or omissions, or numbers which, if up to three of them were changed, would be the numbers 1 to 18 without repetitions or omissions.
**This means, for example, if there are nine candidates, a ballot paper would be informal if it did not have either the numbers from 1 to at least 8 marked without repetitions or omissions, or numbers which, if up to two of them were changed, would be the numbers from 1 to at least 8 without repetitions or omissions.
Note: The numbers on a ballot paper are never actually changed to ensure formality. In addition, a formal vote will only be counted until the point at which the voter's intention becomes unclear, for instance to the point of the numbering sequence where it is no longer sequential.