Celia Wade-Brown’s victory in the mayoral race was not as substantial as first seemed. Using election-night figures, her lead over her nearest rival, John Morrison, was 2284 votes, but she had only 1142 votes more than the 25,712 votes neededfor an absolute majority.

In the 2010 election, WadeBrown won with an absolute majority of just 176 votes. Unlike 2010, when she trailed Kerry Prendergast in the vote count until virtually the end, in 2013 Wade-Brown led all the way, steadily picking up preferences from other candidates as they were eliminated.

Under the preferential voting system used to elect the mayor in Wellington, first-preference votes are counted for each candidate and the bottom-polling candidate is eliminated. The second preferences for that candidate are then added to the voting totals of the remaining candidates.

That sequence is called an iteration. The process is repeated until a candidate has an absolute majority of all the votes cast.

In the 2013 election, the lowest-polling candidate was Karunanidhi Muthu, with 919 first-preference votes. Jack Yan got more of Muthu’s second preferences than any other candidate. After the second iteration Rob Goulden, now with 2590 votes, was the next to go. Morrison got more of Goulden’s preferences than Wade-Brown – 703 to 567 votes.

Some voters do not express preferences for all candidates. There is no legal requirement to do so and the number of candidates for wham voters express a preference varies – 204 of the 919 people who gave a first preference to Muhtu, for example, did not give preferences to another candidate.

Of the voters who voted one or two for Muthu or Goulden, 15 per cent didn’t express a further preference. And 33 per cent of those backing Nicola Young, Muhtu or Goulden did not give a preference to Yan, Morrison or Wade-Brown.

Nicola Young was eliminated on the fourth iteration. Her 5069 votes (including second and third preferences) went to Morrison over Wade-Brown by 1651 votes to 1195 votes. This was fewer votes than might have been expected, given Young and Morrison were broadly from the centre right.

It was the redistribution of Yan’s totaf of 9915 votes that again made the difference for Wade-Brown. The split of his votes delivered victory to her over Kerry Prendergast in 2010.

In 2013 it happened again, but interestingly the margin was not as large as in 2010. This time, Yan’s preferences went 3899 to Wade-Brown and 3237 to Morrison. This was a much narrower margin than might have been expected, given that Yan and Wade Brown are broadly from the centre left.

In 2010 the numbers were 3459 to Wade Brown and 1806 to Prendergast – a much bigger mar

In- 2013, 2279 voters who went for Yan did not express a preference for either Morrison or Wade-Brown – more than Wade-Brown’s lead over Morrison, and more than twice her absolute majority.

Two interesting features of voter behaviour in other countries with preferential voting systems are the role of parties and the disciplined behaviour of voters.

In Australia and Germany, for example, voters are quite comfortable with being told how to express their preferences to maximise support for their party or candidate, even where they have to vote counter intuitively.

Parties routinely make deals for their supporters to give their second and third preferences to like-minded parties, often in return for policy concessions or positions in government – and these recommendations are generally followed.

There are some examples of strategic voting in general elections in New Zealand – Epsom and Ohariu in the last few elections, Wellington Central in 1996 – but it is rare at a local government level.

It’s just not part of our political culture, although in the last two elections Green voters have clearly been more disciplined and the party more organised than the centre right in local government elections in Wellington.