This Labour Party election breaks new ground in New Zealand politics. For the first time, the leader of a major party in Parliament will be chosen by an electorate larger than the party's MPs. Paid-up members of the Labour Party and delegates of its affiliated unions will together have more votes than Labour MPs. The consequences are unknown.
Quite possibly, Labour MPs will not get the leader most of them want. The new rules were adopted by the party last year as a response to the caucus' choice of David Shearer over David Cunliffe who was a stronger performer in front of the party. That is not to say Mr Cunliffe is a certainty to carry the votes of members and unions over the coming weeks - he faces an equally effective speaker in Grant Robertson - but it is quite possible that most of the MPs will prefer Mr Robertson and Mr Cunliffe will be elected.
The third candidate, Shane Jones, could be the best public performer of them all but he will struggle to win the hearts of party activists or unions.
He is casting his net wider than Labour's modern academic and feminist membership, appealing to the "smoko rooms" of yesteryear.
His differences with the Green Party are well known and of the three candidates he may have the most difficulty observing the party's hopeful prohibition of direct criticism of each other.
If Labour is looking for an open debate that will test the mettle of potential leaders, it will need to be prepared for some dissension.
The party's new rules will give New Zealand its first experience of an election akin to a presidential primary in the United States. Like a US primary, it will be a very public campaign for the votes of a partisan electorate. Candidates will be under pressure to take more extreme positions than they would in a general election.
This carries risks not just for Labour but for the good government of New Zealand if the consensus between the two major parties on economic fundamentals is undermined by a leader's public commitments in a campaign for the party's vote.
There is good reason to confine these elections to a party caucus. MPs are generally well briefed on policy issues and aware of the national interest. They also work closely with leadership contenders and are best-placed to assess their character and capabilities.
But there are virtues in giving ordinary party members a vote for its leader. It should boost party membership which has been flagging for a generation now. In fact Labour has to be careful that its election is not white-anted by opponents or extremists who join the party just in time to vote. To be eligible, voters must have joined by midnight on the day the election was triggered (last Thursday) and they have until September 6 to pay this year's dues.
Their votes will count for 40 per cent of the result as will the caucus vote, with delegates of affiliated unions carrying a possibly crucial 20 per cent.
The unions are under party orders not to vote as a bloc, which might ensure the party does not suffer the experience of its British counterpart. British Labour's current leader, Ed Miliband, was elected with union votes despite MPs' preference for his brother.
The machinery for union voting in New Zealand Labour's election is fiendishly complicated. Votes may be cast by a union's national conference delegates or its eligible membership. It is up to the union to decide its procedure, which must be approved by Labour's ruling council.
It might not be the easiest or best way to find the right leader - but it will be interesting to see what happens.