Where is STV used?
Ireland, Malta, Tasmania, the Australian Capital Territory and Australia's federal upper house, the Senate. STV is used for the election of New Zealand's district health boards. It is also used by six New Zealand territorial local authorities including the Kaipara District Council and the Wellington and Dunedin city councils.
What are the advantages of STV?
Despite its complexity, STV ticks a number of boxes. Depending on its design, STV is broadly proportional. Yet, it maintains a direct link in terms of accountability between MP and voters, who effectively have a greater say over which MPs get elected from which party, thereby reducing the power of party bosses.
For example, in a five-member electorate, a party will put up at least three candidates. They might represent different factions within their party. Voters can also distribute their preferences to candidates from different parties.
There are therefore fewer "wasted votes" because most voters' preferences are likely to have had an influence in the election of at least one MP from a multi-member seat.
Voters have a choice of MPs to whom they can take any problems.
STV handicaps candidates and parties with extreme views. While they may secure a slice of first preferences, they are unlikely to win sufficient second preferences from mainstream voters to get elected. All this makes STV the favourite option of experts on voting systems.
What are the disadvantages of STV?
It would be goodbye to a great New Zealand tradition - sitting in front of the TV on election night waiting to see who will be running the country for the next three years.
Computers might be able to forecast the final results. But the official vote count might take some days to reach a final verdict.
A far more significant disadvantage is that the intense competition for votes within an electorate can lead to MPs doing little else but constituency work rather than concentrating on national issues.
This "clientelism" can even see MPs refusing to back unpopular measures until they are sure constituency rivals from their own party are doing likewise.
In Ireland, this behaviour is seen as a drawback of STV, although some political scientists argue the behaviour is more down to the Irish political culture than the mechanics of the voting system.
The establishment of multi-member electorates would mean some would be very large in geographic area.
That might pose special problems in sparsely populated regions like the West Coast. As with all types of preference voting, there is a risk of "donkey voting", that is, apathetic voters ranking candidates in indiscriminate fashion without thinking - especially if voters have little knowledge of the candidates or cannot be bothered trying to understand how STV works.
