Choosing the next Government is not the most important decision voters will make in November, Electoral Reform Committee chairwoman Amy Adams says.

Rather the decision voters make in the MMP referendum will have a longer term impact on New Zealand's future, the National MP for Selwyn said. She was at a National Party event at the Te Kawau Memorial Recreation Centre in Rongotea last night explaining the options voters will face at the election.

"What we decide in this referendum will shape our Parliament ... for many years to come," Mrs Adams said. It was important voters started thinking about the referendum now, and taking time to learn about the different voting systems on offer, she said.

She was concerned the debate on the electoral system would be lost among the hype of the Rugby World Cup and the election campaign which follows straight after.

At the last election, Mrs Adams said a survey showed only 55 per cent of voters understood how MMP, used since 1996, worked.

Voters had two decisions to make in the referendum, Mrs Adams said. The most important was the question of whether to keep MMP or change to an alternate system.

The other decision was regarding a preferred alternative voting system from four options: first past the post, preferential voting, single transferable vote and supplementary member.

If a majority opted to keep MMP, then the voting system would be reviewed by the Electoral Commission to see whether MMP could be improved.

If the majority voted to change MMP, then a second referendum would be held in 2014, pitting MMP against the alternative method that got the most votes in November.

Last night Mrs Adams ran through those four systems, and described the type of Parliament they would likely lead to:

Under first past the post, 120 electorates would exist, with the Government being the party, or coalition, that wins the most electorates.Supplementary member was similar to MMP in that voters had two votes; 90 seats are electorate seats, and 30 list seats are shared between parties based on their share of the party vote.

Preferential voting would have 120 electorates. The system differs from first past the post because voters rank candidates similar to how they do in district health board elections.Single transferable vote is similar but the 120 MPs would be divided up among between 24 and 30 electorates. Each electorate has several MPs, with candidates ranked by voters.

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