Prime Minister John Key has plunged in personal popularity dropping by 7.3 points and Labour leader David Cunliffe has jumped by 3.9 points, in the latest Herald DigiPoll survey.

New Zealand First leader Winston Peters has also risen.

Mr Key is still well above either of them as preferred Prime Minister on 61.6 per cent.

But it is his second lowest rating in a Herald DigiPoll survey since becoming Prime Minister in November 2008.

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Mr Cunliffe has risen from 14 per cent to 17.9 per cent, the best personal rating he has had in DigiPoll since taking over the Labour leadership in September last year.

It is just 0.6 points away from the best DigiPoll rating of his predecessor, David Shearer, who was 18.5 per cent in March last year.

Polling began on Thursday last week in relatively clean air - six days after Judith Collins resigned from the cabinet.

An inquiry will determine whether Ms Collins sought to undermine former Serious Fraud Office head Adam Feeley while she was Minister of Justice, as suggested in an email from Whale Oil blogger Cameron Slater.

Since her resignation, there has been far less coverage of the Dirty Politics books which was denying oxygen to Opposition parties the election campaign.

A gender breakdown in support suggests that Mr Key has lost a lot of support from men.
In last week's campaign poll, 74.8 per cent of men supported him. This week 65.4 per cent of men support him.

Last week 11.9 per cent of men support Mr Cunliffe. This week 17.1 per cent support him.

Mr Peters last week had the support of 5 per cent of men. This week he has the support of 8.5 per cent of men.

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Major events during the polling period include two televised small leaders debates, The Press debate between Mr Cunliffe and Mr Key, Mr Cunliffe's visit to the West Coast and National saying it will introduce modest tax cuts in April 2017 if conditions allow.

PREFERRED PRIME MINISTER

John Key 61.6 (down 7.3)
David Cunliffe 17.9 (up 3.9)
Winston Peters 7.8 (up 1.4)
Russel Norman 3.3 (up 1)
Metiria Turei 2.4 (up 1.5)
Helen Clark 1.3 (down 0.5)
Jacinda Ardern 1.1 (up 0.4)
David Shearer 0.7 (down 0.5)
Colin Craig 0.6 (up 0.2)