The final specific issue being looked at under the MMP Review is the effect of population growth on the proportionality of Parliament.
First we need to look at how the size and number of electorate seats are decided in New Zealand (I didn't know this before today and it is quite interesting). Under the Electoral Act 1993 the South Island must have sixteen electorate seats. You divide the South Island general electoral population (920,999 based on the 2006 census) by sixteen to get the South Island quota (57,562). Then divide the North Island general electoral population (2,690,437) by the SI quota to get the number of electorate seats for the North Island (47). You apply the same maths to the Maori electoral population to get the number of Maori seats (7).
As the North Island population continues to grow disproportionally to the South Island it means that more electorate seats are needed (this is likely to be more pronounced after the next census which will likely show a drop in the South Island population). Where do these seats come from? They are taken from the list seats. Since 1996 the number of electorate seats has gone from 65 to 70, list seats from 55 to 50. At some point in the future this will affect the proportionality of Parliament.
As a keen supporter of MMP and proportionality I think we need to cap the number of electorate seats, at 72 for example (I'm sure some math wizz out there can work out the perfect number). The number of people per electorate would rise but I don't see that being a problem in the foreseeable future. Currently we have 57,562 people per electorate, in Australia it is between 85,000 and 100,000, the UK is 68,175 and in the US it is nearly 700,000.
I believe we could happily live with electorates of up to 100,000 people before we need to look at enlarging the size of Parliament itself.