Labour has almost halved National's lead in the Herald-DigiPoll survey.
National has slipped by 4.6 percentage points in a month memorable for slip-ups and lacklustre performances by leader John Key.
But the party is still ahead - it has the support of 49.9 per cent of decided voters, and could still govern alone with 63 MPs.
Labour's fight-back has lifted it 2.8 points to 39.3 per cent in a month of carefully designed publicity hits for the Government, including a snap move to try to keep strategic assets such as Auckland Airport in New Zealand control.
The gap between the two big parties is now 10.6 percentage points, much closer than the 18-point lead National had on Labour early this month.
Prime Minister Helen Clark and Mr Key are virtually neck and neck as preferred Prime Minister after a small rise for her and a small drop for him.
He is preferred by 45.9 per cent of decided voters and she is preferred by 45.6 per cent.
New Zealand First leader Winston Peters is up slightly to 3.9 per cent, which may be heartening to his party, which has fallen to 1.1 per cent, its lowest Herald-DigiPoll rating since the 2005 election.
Mr Key started the year strongly with a "state of the nation" speech on youth crime that delivered policy detail after a summer news diet heavy on youth crime stories.
Helen Clark broke with tradition and gave her own state of the nation speech in a bid to stop him setting the political agenda in January.
But her speech, announcing a new education and training leaving age and school apprenticeship scheme, was viewed as stodgy by comparison.
It has since been repackaged as the "Schools Plus" scheme.
Her statement to Parliament in February focused on housing affordability and social services delivery, but it still did not give Labour the lift it wanted.
Mr Key's good start was reflected in a lift in ratings for him and his party. He overtook Helen Clark as preferred Prime Minister for the first time since May last year and National stretched its lead to 18 points.
But March was a bad month for Mr Key, and that shows in the poll.
He was tagged "slippery" after failing to articulate a concise response to Labour's move to keep Auckland Airport in New Zealand control.
He made errors explaining his party's position on Treaty of Waitangi settlements.
And National has had to contend with a potential association by marriage to hard-right politician Sir Roger Douglas, who is returning to active politics as an Act candidate.
If this month's poll figures were translated into votes, the Greens and New Zealand would not get back into Parliament unless they won an electorate seat.
The Greens are on 3.9 per cent (down 0.5); New Zealand First is on 1.1 (down 1); the Maori Party is on 3.7 (up 2.2); United Future is on 0 (down 0.4); and Act is on 1.1 (up 0.7).
Registering for the first time is the Kiwi Party of MP Gordon Copeland (formerly of United Future), which has 0.4 per cent support.
Assuming party leaders keep their electorate seats, the poll figures would give National 63 seats, Labour 50, the Maori Party 5, and Act, the Progressives and United Future one each.
The Maori Party party vote has picked up to the extent that it would be entitled to five MPs, adding a list MP to the four electorate members it has at present.
The issue most likely to influence voters is still tax cuts, followed by the economy, law and order, hospital waiting lists and global warming.
* The poll of 750 respondents was conducted between March 6 and 27 and has a margin of error of 3.6 per cent. The percentage of undecided voters was 10.9.