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.