Andrew at Grumpollie does some calculations:
Each wave surveys a fairly large sample of voters and a small sample of non-voters. So i was having fun, and I started to wonder what would happen if all the non-voters with a party preference had got out and voted on Election Day.
There are a bunch of caveats to this analysis, including the small sample size and how representative the sample of non-voters was. BUT, if we assume for a moment that the data were broadly representative, then inspiring all non-voters to get out and vote wouldn't have had a massive impact on the 2011 result.
Andrew calculates that if every non-voter with a preference had voted, then National would have gone up 0.1%, Labour up 1.7%, Greens down 1.0% and NZ First down 0.8%.
So my take on this is that just inspiring a larger turnout won't necessarily help Labour. In 2008 it would have,but in 2011 it wouldn't have.