There has been a boost in support for the Labour Party, the latest ONE News Colmar Brunton poll suggests.
The opposition party has moved to within 11 percentage points of National.
When combined with the Green Party, the centre-left bloc is now ahead of National for the first time since the election in 2011.
The poll indicated Labour had the support of 34% of respondents - an increase of two points - with National slipping three points to 45%.
The Greens held steady at 12%.
Minor parties making gains this month include the Maori Party up one point to 3% and the Conservatives gaining one point to 2%.
New Zealand First has slipped one point to 2%, while Act and Mana held steady on 1%.
With these numbers National would have 56 seats in Parliament, two fewer than the centre-left bloc of Labour, with 43 seats, and the Greens, with 15.
Act, United Future and Mana would have one seat each. The Maori Party, with four seats, would most likely hold the balance of power.
This assumes all current electorate seats are held.
As to whom poll respondents would prefer to be prime minister, support for John Key was down one point but still strong at 44%.
David Shearer's support was down one point to 12%, the second month in a row that his numbers have dipped.
Winston Peters was steady on 4%.