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Mr Key is still well ahead of Labour Leader David Cunliffe but Mr Cunliffe has jumped by 4.1 points.
Mr Key is preferred Prime Minister by 64.8 per cent, compared with Mr Cunliffe on 14.6.
The figures do not necessarily reflect the party vote standings which will be released in tomorrow's Herald.
National's party vote is traditionally a lot lower than Mr Key's personal poll rating and Labour's party vote ratings have been habitually higher than the personal ratings of Mr Cunliffe and his two predecessors, David Shearer and Phil Goff.
Mr Hager released his book on August 13 and polling began the day after that day.
Issues arising from the book have dominated political coverage in the past week, namely what dealings Mr Key, his advisers and Justice Minister Judith Collins had with Whaleoil blogger Cameron Slater, who is renowned for his attack blogs.
Mr Key has been on the defensive continuously, initially over Ms Collins sending personal details to Mr Slater of a public servant she suspected of leaking information to the media. The public servant was viciously attacked on the blog and received death threats.
However the focus has turned to Mr Key himself and the issue of the SIS giving Mr Slater preferential treatment in OIA requests for documents that would be embarrassing for then Labour leader Phil Goff.
New Zealand First leader Winston Peters is the third most preferred Prime Minister at 5.1 points, and Greens co-leader Russel Norman is the fourth most popular at 3.5 per cent.
Mr Key's results have landed him back at where he was for most of this year in the mid 60s. His high rating in July followed a Budget which was well received and a particularly low patch for Mr Cunliffe following the "sorry for being a man" comments.
Mr Cunliffe's popularity was highest last September, with 16.8, straight after the Labour leadership contest and they steadily declined to 10.5 last month before rebounding today.
The Herald will be polling weekly up to the election on September 20.
PREFERRED PRIME MINISTER
John Key 64.8 (down 8.5)
David Cunliffe 14.6 (up 4.1)
Winston Peters 5.1 (down 0.4)
Russel Norman 3.5 (up 1.5)
David Shearer 1.5 (down 0.7)
Helen Clark 1.4 (up 0.9)
Jacinda Ardern 1 (-0.4)
Phil Goff 1 (up 0.7)
- By Audrey Young of the NZ Herald