The election campaign is in knife-edge territory after a further poll showing Labour has made massive gains on National - but National leader Bill English still managing to hold his own against the onslaught of Labour's Jacinda Ardern.

The Newshub Reid-Research poll had Labour up 6 points in three weeks to 39.4 - four points behind National which was down one point on 43.3 per cent.

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That was better news for National and English than the One News Colmar Brunton poll on Monday night which had Labour on 43 per cent, ahead of National on 41.

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However, both polls also have Ardern neck and neck with English as preferred PM - in the Reid Research poll, Ardern was at 29.9 per cent while English was a whisker ahead at 30.1 per cent.

It was a meteoric rise for Ardern - but is also English's best result in the poll. He was up almost three points from the previous poll, indicating National voters are rallying behind him.

The poll comes as English and Ardern prepare for the second head to head debate tonight on Newshub at 8.30pm. The TV1 debate last Thursday gave them a chance to warm up.

The smaller parties, NZ First and the Greens, had both suffered in the poll - casualties as voters get in behind the larger parties and the lack of oxygen as the race closes in.

NZ First was down almost three points to 6.6 per cent and Winston Peters had dropped as preferred PM to 6.9 per cent - but would still be in the kingmaker position on those results. The Greens had also dropped down two points to 6.1 per cent - and Labour would need both the Greens and NZ First to get over the line while National would need NZ First.

That was at least above the five per cent threshold to get into Parliament but without much of a buffer.

The Opportunities Party was on 1.9 per cent, the Maori Party on one per cent and Act on 0.6 per cent.

The poll of 1000 voters was taken from August 22 - 30 and has a margin of error of +/- 3.1 per cent. It includes an internet panel of 250.

Despite the result showing National was holding up, English and National will be wary of their vote eroding as Ardern's momentum shows no signs of stopping.

It was National's lowest result in that poll in more than a decade. The poll results equate to Labour picking up support of 490,000 new voters in the month since Ardern became leader - an astonishing turnaround. Labour was on 24.1 per cent in July.

As a counter, English has waited for what he called the "stardust" to settle and minds to turn to policy.

English has pushed his economic credentials, using the last debate to try to hammer Labour for what he described as a "vague" and "confused" position on taxes.

However English's own economic management has been criticised as uninspired by the boss of one of New Zealand's most successful companies.

In a video interview for the Herald's Mood of the Boardroom Election Survey, Mainfreight chief executive Don Braid said the country was "being run by a couple of accountants, rather than visionaries".

Braid said National's sudden decision to invest in the country's infrastructuire after three terms and just ahead of an election looked unconvincing to voters.

He didn't see any vision from English, whereas Jacinda Ardern was having "the John Key effect for the Labour Party".

On the campaign trail yesterday English moved on another of National's traditional planks - law and order. He set out plans for a crack down on gangs including giving Police new powers to search the property of gang members without needing a warrant or a good reason for a search. That announcement also included an $82 million package to increase drug rehabilitation.

Ardern was also focusing on one of Labour's key campaign planks - housing. She spent the morning in friendly Labour territory at the Avondale markets before announcing policy to make it harder for landlords to evict tenants and banning letting fees, saying long-term secure rentals were critical for families which could not afford to buy homes.