Prime Minister John Key - smiling all the way to a big poll lead.

The latest Roy Morgan political poll has the John Key-led National Party's support jumping 7.5 per cent to 50.5 per cent.

It follows the July survey in the volatile series that put the combined Labour-Green vote ahead of National.

In the latest survey that was reversed with Labour on 27 per cent (down five) and the Greens 11 per cent (down two) - a combined 38 per cent despite recent bad economic news including a weakening dairy price and the failure of state-owner coal miner Solid Energy.

National partners' backing was unchanged, with the Maori Party on 1.5 per cent, ACT 0.5 per cent and United Future on zero.

Support for NZ First was up one percentage point to 8 per cent.

Internet-Mana continued to languish at 0.5 per cent and the divided Conservatives shed all their support to record zero, down from 1.5 per cent in July.

The latest Roy Morgan Government Confidence Rating was 120 points, up two, with 54 per cent of electors saying the country was "heading in the right direction" compared to 34 per cent saying it was heading in the wrong direction.

The survey of 892 was taken by landline and mobile phone in August. Of those polled 5.5 per cent did not name a preferred party.

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