The South Island will suffer if the number of MPs is cut to 99, says Green Party Co-Leader and MP Rod Donald.

A referendum is to be held during the general election about whether to reduce the size of parliament from the present 120 seats to 99.

"Clearly any reduction in the number of South Island electorate seats, currently 16, would mean even larger electorates and less representation," Mr Donald said.

If only list seats were cut this could threaten the level of South Island representation. This is because the number of electorate MPs rises according to a formula based on the ratio of the number of people living in the North Island compared to the number in the South Island and the number of list seats is reduced to compensate for the number of extra constituency seats.

Already, because of the population growth in the North Island, at this year's election there are two more constituency seats than for the last election, and two fewer list seats.

"If this trend continues, as expected, then the number of list seats would soon fall below 30, increasing the likelihood of a Party winning more constituency seats than its share of the Party vote. When this happens a Party is, obviously, entitled to keep all its constituency seats," Mr Donald said.

"Not only would that mean there would be more than 99 seats for that parliament, but proportionality would also be lost because of this overhang."

To avoid undermining our system of proportional representation, either the number of South Island constituency seats would need to drop below 16, or the population size in each North Island seat would become increasing larger than in the South. Either option would be very unpopular, Mr Donald said.

The physical size of the South Island electorates already makes them very difficult for MPs to service.

There are currently 28 constituency and list MPs representing the South Island - roughly equivalent to its proportion of the population of New Zealand.

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