A Roy Morgan poll shows a fall in support for National and a rise in support for Labour.

Support for National fell 3.5 points to 43 per cent in the July poll while support for their coalition partners was unchanged with Maori Party on 1.5 per cent, ACT on one per cent and United Future on zero.

Labour rose five points to 30.5 per cent, while support for the Greens was down 0.5 per cent to 13.5 per cent.

Support for New Zealand First was down one point to eight per cent.

Executive chairman Gary Morgan says this year's election is on a knife's edge and New Zealand First are likely to decide the government:

"New Zealand's election in two months is now too close to call," he says.

The drop in support for National follows the scandal involving National MP Todd Barclay.

The poll is at odds with other recent polls.

Yesterday Newshub reported an internal Labour poll leaked to it has Labour with 26 per cent support.

That compares with 26.4 per cent in latest Newshub Reid Research survey and 27 per cent in the latest Colmar Brunton poll.

National was on 47.4 per cent in the Reid Research poll and 47 per cent in the Colmar Brunton poll.