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.