National has taken a 6 point dive in the latest Roy Morgan poll, below the combined support of Labour and the Greens at 45.5 per cent.
New Zealand First would hold the balance of power and decide the Government, assuming that the Mana, Maori Party, United Future and Act keep at least one electorate.
New Zealand First would also hold the balance of power if the assumptions extend to the Conservatives winning an electorate seat and the proposed alliance between Mana and the Internet Party going ahead.
The next election is on September 20.
The second-term National-led Government has been mired in controversy over ministerial behaviour. Maurice Williamson resigned last week after documents obtained by the Herald showed he had interfered in a police prosecution against a wealthy property developer and National Party donor.
Justice Minister Judith Collins has been subject to months of pressure from the Opposition regarding her involvement with her husband's export company during a justice-related trip to China last year and she has just begun a week's leave.
Labour is up by 2.5 points to 31 per cent, and the Greens are up 3 points to 14.5 per cent.
New Zealand First is up by 0.6 to 6 per cent.
Among National's current support partners, the Maori Party is unchanged on 1 per cent, Act is unchanged on 0.5 per cent and United Future goes from zero to 0.5 per cent.
Mana is unchanged on 1 per cent but the Internet Party is on 1.5 per cent.
The proposed alliance with Mana would mean two Internet Party MPs in the next Parliament.