KEY POINTS:
National has hit its highest rating - 51.3 per cent - in a Herald-DigiPoll survey.
Translated to votes, that would mean the party could govern alone.
Labour has slipped to 13.2 points behind National, increasing last month's 12.4-point gap.
But Prime Minister Helen Clark remains confident Labour will win the next election.
The poll coincided with the first anniversary of John Key taking the National Party leadership after the resignation of Don Brash.
Mr Key said it would be hard for National to go any higher in the poll. Its previous high point was 51.2 per cent in the October poll.
And the party would have to work hard to maintain such a high level of support.
"One in two New Zealanders are indicating they will vote for National," he said.
"There is obviously never a limit, but there comes a point where it is harder when you are higher up to increase your vote, and maintaining your level of support is important."
A spokeswoman for Helen Clark said she was satisfied that Labour's polling was stable and close to the support that put it into government in 1999.
She regarded it as a very strong basis on which to go into election year.
New Zealand First is polling under 5 per cent and would disappear altogether unless leader Winston Peters wins an electorate seat.
But a jump in his rating as preferred prime minister to 8.1 per cent will buoy the party's hope that that is achievable.
The figure is 3.1 points above his October rating and coincides with his high-profile trip to North Korea and Washington and a party conference at which he attacked racial separatism.
Helen Clark is still well ahead of Mr Key as preferred prime minister, favoured by 48.7 per cent (down 2.1) to Mr Key's 36.7 per cent (down 0.6).
Translated to seats, and assuming party leaders with seats retain them, the poll results would give National 65 seats in a 121-seat Parliament, Labour 49, the Maori Party 4 and Jim Anderton's Progressives, Rodney Hide's Act and Peter Dunne's United Future would have one each.
The Greens' support has plunged in Auckland, and they would have no MPs in the next Parliament.
They fell below the 5 per cent threshold half-way through the year, then climbed back up.
Last month, the party's support fell to 5.4 per cent, but it had strong support in Auckland, where it reached 8.3 per cent. But outside Auckland, the figure was 4 per cent.
This month, the total Green support is 3.5 per cent, but the party had only 0.9 per cent in Auckland, against 4.7 per cent elsewhere.
Greens co-leader Jeanette Fitzsimons said that if several polls showed the same trend she would be worried, but she was not worried about one poll.
"The thing that hurts us most is a squeeze between the two big parties and that is certainly going on at the moment.
"The more people get agitated about National-Labour, Labour-National, the more the vote drains away from the Greens."
* The poll of 912 decided voters was conducted between November 8 and Monday. The margin of error was 3.2 per cent.
