Support for the ruling National Party has fallen to its lowest level since the 2011 election, and a potential Greens/Labour coalition is now polling higher.

National was at 44 per cent support in a Roy Morgan Poll taken between July 23 and August 5, down 3.5 per cent from the last poll taken between July 9 and 22.

Support for Labour rose 2 per cent to 32 per cent, while backing for the Greens rose 3 per cent to 14 per cent.

National's result is at its lowest level since the election and the 12 per cent lead over Labour is the closest gap between the two main parties since just after National won government in 2008.

If an election were held now it would be too close to call and depend on the voting results of minor parties and whether they crossed the 5 per cent threshold to win list seats.

Support for National's coalition partners was little changed with the Maori Party on 2 per cent, ACT on 0.5 per cent and United Future on 0.5 per cent.

Support for the Conservative Party, which has taken a strong position against law reform to allow gay marriage, has fallen 1.5 per cent to 1.5 per cent.