It is bad news for National in the latest Colmar Brunton poll, falling two points to 39 per cent, the first time it has been in the 30s since 2005.
Labour is steady on 43 per cent and would be able to form a government with New Zealand First, which is up 1 to 9 per cent.
Labour overtook National in the same poll last week, 43 to 41, and Ardern drew just ahead of Bill English as preferred Prime Minister, 34 per cent to 33 per cent.
National's polling has been steadily declining since May when it polled 49 per cent after a strong budget.
In two July polls it polled 47 per cent. In mid August it polled 44 per cent and last week fell to 41 per cent.
Just prior to the polling period, Winston Peters revealed that he had been overpaid superannuation for seven years and had repaid it.
He also accused National of having leaked the information about his pension to the news media and it was confirmed that Paula Bennett and Anne Tolley had been told by officials under the no-surprises policy.
The full results: Labour 43 (no change); National 39 (down 2); New Zealand First 9 (up 1); Greens 5 (no change); Maori Party 2 (up 1); Opportunities Party 2 (up1), and Act 0.1 (no change).
Translated to seats in the House, Labour would have 53 seats, National 48, New Zealand First 11, Green Party 6, Maori Party 2, and Act 1 (assuming that Act and Maori retain an electorate seat).
With 61 seats needed to form a government, Labour and New Zealand First could do it with 64 seats.
Labour could also just do it without New Zealand First, but with the Greens and the Maori Party.