The process leading to a binding referendum on mixed-member proportional representation has been set in train. That much is welcome. The public voted in the electoral system and should decide its future. Less encouraging is news that the Cabinet is discussing the wording and organisation of the referendum. Since MMP's introduction at the 1996 election, self-serving politicians have been loath to let it slip from their grasp. Now the Key Government is reserving critical decisions to itself, rather than turning them over to the Electoral Commission.

This is a well-trodden path, and a reason the National Party's campaign pledge to hold a referendum found widespread favour. When MMP was adopted, a review was promised after two elections. The public expected a referendum but what eventuated was not even an impartial examination by the Electoral Commission. Instead, a special parliamentary committee, compromised by the preservation instincts of the minor parties, found nothing amiss. The public has nursed a justifiable grievance ever since.

Ironically, that sentiment has little to do with the overall working of MMP. After a shaky start, the system has largely fulfilled the expectations of its supporters in delivering representative government. Most significantly, it has halted an approach by two successive governments that smacked of a headlong rush and owed little to consultation or consensus. Not only did those governments get far in advance of the public but their executives often raced well ahead of their own caucuses. MMP has, as promoted, ensured that the unbridled power sponsored by the first-past-the-post system can no longer reside in the hands of a single party.

And if politics has not been transformed, as some may have imagined, nothing drastically bad has emerged from five elections to suggest MMP has been a mistake. Several things could, however, conspire to make the referendum closer than it should be. One is the sort of collective memory failure that consigns the events of the 1980s to the dustbin of irrelevant history. Another is aspects of MMP that continue to irk many people. Most notably, these include list MPs and the disproportionate influence of small parties. But the referendum requires voters to wield a broader brush and to assess MMP against other electoral systems. It is not about fringe irritants, even if these should be addressed at some stage.

More widely, public cynicism about political conduct could spill over into distrust of MMP. That is why the Electoral Commission should be orchestrating the referendum. As in all such polls, the framing of the question will be extremely important. It should be handled by a neutral party, not the Cabinet. Likewise, the referendum schedule should be free of political overtones. Those eager to see MMP dumped, including former Telecom chairman Peter Shirtcliffe, want a single-stage definitive poll next year, the outcome of which would be applied at the 2011 election. A wiser course is a two-stage process, with a first referendum, held at the time of the next election to save costs, asking voters whether they wanted a change. If change is favoured, a second poll on the options would be held before 2014.

The Prime Minister seems inclined to pursue this more measured approach. But, in accord with the anti-MMP camp, he has also signalled his support for supplementary member voting, the halfway house system that would see most MPs elected under first-past-the-post and a smaller number on party lists. Once again, there is the threat of political intrusion. The only solution is for politicians, finally, to step away from the process.