Pollsters say the difference between two political polls released last night is so great one of them has to be wrong.
Simon Bridges and Jacinda Ardern Photo: RNZ
The 1 News Colmar Brunton poll saw National up four points to 44, and Labour down six to 42, while the Newshub Reid Research poll has Labour able to govern alone on 50.8, and National down to 37.4.
David Farrar Photo: SUPPLIED
Curia Market Research principal David Farrar said the differences between the two went beyond the margin of error or differences in methodology.
He said the 1 News poll uses landlines and cellphones, while Newshub's uses landlines and an internet panel.
Mr Farrar said they were also taken over slightly different periods. The 1 News Colmar Brunton poll was taken between 4 and 8 June, and Newshub's spanned 29 May to 7 June.
"The methodology differences can't explain differences of this magnitude.
"It could be that if there were a big slide in the government's fortunes after the Budget, that would explain some of the difference.
"But to be honest the differences are so great that a timing difference of six days, that's not credibly all of the answer", Mr Farrar said.
The reliability of polls has come under scrutiny following upsets like Brexit, the 2016 US election, and last month's Australian election - where polls had consistently predicted a Labor victory.
Mike Williams Photo: Kim Baker Wilson
Former Labour Party president Mike Williams said it was no wonder the public's faith in polls was shaken.
"The credibility of polls must be falling away, and it must be damaging the whole industry."
"I think the problem is it's getting very difficult to capture a representative sample of voters", he said.
Mr Williams said the methodology of these polls needed to reviewed as relying on landlines was no longer viable.
Australian electoral analyst Dr Kevin Bonham said the pollsters themselves could be adding to the misguided forecasts.
Dr Bonham said pollsters were reluctant to report outlier results.
"Nobody wants to be the one poll that got it wrong while everyone else got it right because that's a bit of a reputational disaster.
"There have been a number of cases around the world of this thing, often known as 'herding' happening, where all the polls near to the election are very similar to each other", he said.
Dr Bonham said no one should put too much stock in one poll, or one polling company, and trends needed to be seen over time.
