New Zealand will retain MMP as its electoral system, but those who voted to get rid of MMP can still have a say on how it can be improved.

Today's release of the full result of the referendum is expected to replicate the preliminary figures from election night last month - 53.7 per cent in favour of retaining the system, 42.6 per cent wanting it dropped.

But sticking with the status quo triggers clauses in the Electoral Referendum Act which require the Electoral Commission to review MMP.

Those clauses were inserted in the law by the outgoing Justice Minister Simon Power, who said many people wanted to retain MMP - but with some changes.

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Whether that helped swing some voters behind MMP will never be known. But the provision for a review got little publicity.

It is not yet clear exactly how the commission, which is an independent body, will conduct the review.

But the exercise must include public consultation and any recommendations for change must be made to the new Justice Minister by next November.

The referendum act stipulates that the commission will examine five aspects of MMP - plus anything else it thinks needs scrutiny.

However, the review is barred from looking at Maori seats or the number of seats in Parliament.

These two aspects are the subject of a Maori Party-instigated constitutional review agreed to by National.

The MMP review will cover other bugbears in the system:

* The fact that MPs defeated in constituency contests can return through their party's list. Likewise, the increasing practice of list MPs standing as candidates in byelections.

* The rule that removes the 5 per cent threshold if a party wins an electorate seat.

This rule created anomalies, such as in 2008 when New Zealand First won more than 4 per cent of the party vote and remained seatless, yet Act got five MPs through holding the Epsom electorate despite securing fewer party votes.

One option would be to remove this apparent anomaly and lower the threshold, which is also subject to the review, say to 4 per cent.

* The fact that lists are compiled by parties with no voter input. The commission will look at whether to open party lists to voters and allow them to rank their preferences.

* The "overhang", whereby Parliament increases in size when a party wins more electorate seats than its share of the party vote.

* The longer-term problem of ratio of electorate seats to list seats.

As the population of the North Island rises, so the total number of electorates has to rise.

If the number of list seats falls below around 40 per cent - a 48:72 ratio in a 120-seat Parliament - true proportionality of party representation is at risk.

There are now 70 electorates - 63 general seats and seven Maori seats.

The obvious answer is to increase the size of Parliament.

The commission may yet have to recommend an increase, despite it seemingly being barred from doing so.