National has plunged nearly five points to below 50 per cent in the past week and the smaller parties - including a fast-rising New Zealand First - are picking up support, according to the latest Herald-DigiPoll survey.
Prime Minister John Key is expected today to have a symbolic public meeting with Act Epsom candidate John Banks, who on today's poll would take another Act MP into Parliament with him if he won the electorate seat.
If National's polling slips further before the November 26 election, two Act MPs could become important to Mr Key forming a new government.
It is the first time National has fallen below 50 per cent in a Herald-DigiPoll survey since the last election.
Mr Key said through a spokeswoman that the poll underlined that the election "is definitely not over".
"We're working hard for every vote and seeking the support of New Zealanders to form a strong, stable government at this crucial time."
NZ First is within striking distance of the crucial 5 per cent threshold, with just 1.3 points to get there. While Labour has lost support, the popularity of its leader, Phil Goff, has shot up.
The Greens are also on the rise and would have 16 MPs - seven more.
National, on 49.5 per cent support, would just be able to govern alone with 63 MPs- an increase on its present 58.
Based on today's poll, if Mr Banks won Epsom, he would bring leader Don Brash into Parliament as well.
Mr Key is still streets ahead as preferred Prime Minister but has dropped 2.1 points to 68.5 per cent.
Support for Mr Goff in the preferred PM stakes has surged 6.2 points to 17.9.
Having languished in the polls for three years, he is riding to new heights with a strong campaign against National's policy of partial asset sales and with a heavy dose of media training to project a more human politician.
He has also made much of Mr Key's failure to meet a pledge to cut the number of Kiwis moving to Australia.
Labour fell last week to below 30 per cent for the first time in a DigiPoll survey and this week has dipped again by 0.4 points to 28.7 per cent.
The Greens have not only had a 2.5 point jump in a week to 12.6 per cent, but they are heavily favoured to be National's support partner.
The poll put to voters that if National were to lead the next government with just one other party, which one would be best. Just under 39 per cent chose the Greens, compared with 18.9 for Act, 17.8 for the Maori Party and 10.6 per cent for NZ First.
Polling began on Thursday last week, the day after the debate in which Mr Key demanded that Mr Goff "show me the money" over Labour's costings.
Mr Goff presented Labour's fiscal policy the next day, and during the polling period announced Labour's child policy, including an in-work child tax credit to beneficiaries, and extending paid parental leave to six months.
In the same period, National released its law and order policy, including locking up dangerous prisoners indefinitely even after they have completed their sentence.
NZ First leader Winston Peters pledged this week not to go into a coalition. But he said the same thing in 2005, then went with Labour. He was voted out of Parliament at the last election.
The decline in National's support is reflected in the drop in the percentage who believe the government is moving in the right direction - from 62.3 per cent last week to 54.7 - and a rise in the number who think it is not (36.2 per cent, compared to 30.4 per cent).
* The poll of 750 voters was conducted from November 3 to 9. The party vote is of decided voters only and 8.1 per cent of the sample was undecided. The margin of error is 3.6 per cent.