How does preferential voting work and why is it important?

By Jesse Neill.

Most of us know how to cast our vote, but do we really know how this voting system works and why it is important?

We explain how preferential voting systems will work in the upcoming South Australian state election.

For voting in the House of Assembly (lower house) there will be a list of candidates with boxes next to their names.

Voters are asked to rank their ballot from the most preferred to least preferred candidate.

A how-to-vote card for the dignity party in the electoral district of Colton is an example of this process:


Voters can list one preference if they are indifferent to others, however, if their first preference is eliminated they have no other preferences to pass on, this is known as an exhausted vote.

The vote is made invalid if two of the same numbers are listed or any of the numbers are missed in the sequence.

The winning candidate is initially determined by counting the first preferences votes, also known as the primary vote.

A candidate must have more than 50 per cent of the primary votes to win that seat.

If there is no clear winner, then the candidate with the least number of primary votes is eliminated from the count.

Primary votes for the eliminated candidate are re-allocated amongst the remaining candidates according to voters’ second preference.

This process continues as candidates are eliminated and their votes re-allocated until there is a candidate with a clear majority.

If a second preference is given to a candidate that has already been removed from the count, the voter’s third or subsequent preference is used.

This method of preference allocation is illustrated in an example from the parliament Education Office:

Parliament Education Offi.png

This means even if one candidate receives a high percentage of the primary vote – they may be highly divisive and not secure second or third preference votes, meaning other candidates could overtake them.

If a political party wins more than half the seats in the House of Assembly (lower house) then they form government.

Voting in the Legislative Council (upper house), is done through a single transferable vote, which is a similar but slightly more complicated form of preferential voting than that used in the House of Assembly.

As candidates and parties are competing for more than one seat, there is a certain quota to reach rather than a clear majority.

If a party or individual reaches the quota, then the surplus of their primary votes are redistributed to their second preference.

The least popular vote is then removed and redistributed like the instant run-off voting system explained earlier.

This example of how Single Proportional Voting works is from the Electoral Reform Society in the UK.

To calculate the quota: There were 109,525 valid votes cast and three seats to be filled. So, 109,525 divided by four (because we have three seats, plus one), then plus one = 27,382.


  1. John has easily reached the quota so is elected. His surplus votes are transferred based on second preferences.


2. Many of John’s supporters liked his running-mate Mary. She gets enough of the transferred votes to be elected.


3. Mary’s surplus votes are transferred but nobody has enough to be elected.


4. Helen has been eliminated and the votes she won are transferred.


5. Paul is also eliminated


6. It’s now impossible for Sally to win, so Stephen wins the last seat.

While both of these may seem like complicated systems to understand, they are based on determining the fairest outcome possible.

Proportional voting ensures the winning candidate is the most preferred or least disliked, rather than electing a minority winner.

It also means voters can back an independent candidate or minor party without wasting their votes as their preferences are an important part of the election process.

Parties with like-minded policies may also use this to exchange preferences in an effort support each other and encourage a stable two-party system.

This election is shaping up to be one of the most important and tightest state elections in recent history.

This is why it’s important to understand how preferential voting works and provides one certainty for this election: every vote will count.



Image Source: The Advertiser

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