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Tonight's One News Colmar Brunton poll put support for Mr Key as preferred prime minister on 59 percent, up 2 percentage points on last month, and his highest result since he was first included in the poll in 2006.
Support for Labour leader Phil Goff held steady on 8 percent.
The gap between the two major parties also widened, with National remaining on 56 percent while Labour dropped 1 point to 29 percent.
The only other party to make gains was the Greens, up 3 points to 9 percent, while the others lost support and remained below the 5 percent threshold needed to get into Parliament without winning an electorate seat.
National would have enough seats to govern alone if the results were replicated at the November 26 election, with 69 compared with Labour's 36.
Voters' confidence in the economy fell, with optimism down 4 points to 45 percent, and pessimism up the same amount to 35 percent.
The poll of 1008 eligible voters, conducted between September 24 and September 28, had a margin of error of 3.1 percent.