The Herald-DigiPoll survey came closest to predicting the final election result, an analysis of the four major polls shows.

It was the only one of the mainstream polls which did not underestimate National Party support or significantly overestimate the Green vote. Along with the Colmar Brunton survey, all its party vote predictions were within the margin of error.

The Herald looked at the final predictions by four polling companies and compared them to the provisional party vote results published by the Electoral Commission yesterday. Special votes could change the final result.

The Herald-DigiPoll's predictions differed from the actual result by 4.33 percentage points. The Stuff-Ipsos survey was the second most accurate, with a difference of 7.71 points.


The timing of the polls appears to have been a factor. National voters may have rallied in the final few days of the campaign in response to the Internet Party's "Moment of Truth" event, which targeted Prime Minister John Key and his Government's alleged surveillance programmes.

The 3News-Reid Research poll closed on September 15, the day of the event, and predicted National would get just 44.5 per cent and that Conservatives were on the cusp of election at 4.9 per cent. Polls that closed later in the week, on September 17, proved to be more accurate.

The online trading market iPredict, which came closest in 2011, performed more poorly than any of the major polls.