National holds a decisive election advantage over a Labour party struggling to gain traction with voters, successive polls reveal.

Labour's support had dropped to 31.5 per cent - its second lowest level since 1999 - in a Herald digipoll take in August 2011.

That compared to consistently strong backing for National, which stayed steady on 52 per cent support in the most recent poll.

The poor showing for Labour has fuelled speculation Finance spokesman David Cunliffe is gathering support for a leadership challenge.

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Voters have consistently shown a wide gulf in the popularity of Labour leader Phil Goff and National leader John Key.

Mr Goff recently recorded 11.5 per cent of votes as preferred Prime Minister, to 69.7 per cent for Mr Key.

The Green Party has profited from Labour's decline, recording 9.8 per cent support in the August digipoll.

They received the most support among the 40 - 64 age group, which voted Green at a 12 per cent clip.

Meanwhile, New Zealand First leader Winston Peters is still thought of as 'good for New Zealand' by a third of Kiwis, despite nearly three years in the political wilderness.

- NZ HERALD STAFF