The Government's opponents are stepping up their condemnation of Prime Minister John Key's response to the book Dirty Politics.

And they're seizing on results of a ONE News Colmar Brunton snap poll on the public's reaction to allegations in the book by Nicky Hager.

The poll, taken since Dirty Politics was released on Wednesday night, asks voters if they believe the allegations of National Party smear tactics.

Twenty-eight percent said "yes" they did, 43% said "no" and 29% didn't know.

Asked how it influenced their view of the National Party, 4% were positively influenced. But 9% now have a negative view of National, while 82% aren't influenced much.

"Nine percent this close to the election, that's dynamite," said Winston Peters, New Zealand First leader.

Labour leader David Cunliffe says the book's impact has been incredible.

"Over one in 10 New Zealanders is going to change their vote because of that book. That's an incredible impact and it will only grow as people learn more," he said.

Finally, asked if the allegations would now make them more or less likely to vote, 12% said they were now more likely, 1% were less likely and 87% said the book hadn't made much difference either way.

'This is too serious'

The Prime Minister says the poll shows most New Zealanders are focused on the real issues, not "highly political claims" like those made by Mr Hager.

But that's unlikely to stop Mr Key's opponents attacking his handling of the issue.

"He thinks he can can character assassinate Nicky Hager. The problem is Hager didn't write this book, basically. The National Party did," Mr Peters says.

Mr Cunliffe says Mr Key "lied" that he had Justice Minister Judith Collins on third strike.

"And now he doesn't care. He's lied that he has no responsibility for the guy who worked two doors down from him."

The Prime Minister stands by his earlier statements.

"It's got nothing to do with my office," Mr Key said on Thursday.

And he says Mr Cunliffe's accusation hardly seems consistent with his 'Vote Positive' election campaign message.

But Mr Cunliffe says Mr Key, "thinks he can get away with it. Not this time, this is too serious."

Aiming to back up that claim, Labour has released emails it says contradict Mr Key's claim that National wasn't involved in accessing a Labour Party database in 2011.

National is hitting back, saying there's nothing new in the emails.