Peter Dunne says the mood for change that saw him quit politics weeks from the election isn't deep-seated - and Labour's election campaign launch demonstrated a superficial "mood for excitement".

The United Future leader and Ohariu MP said he told Prime Minister Bill English of his decision to quit this morning, after polls showed he was behind Labour's candidate and former Police Association president Greg O'Connor.

Until about three weeks ago the polls were strongly in his favour, Dunne said, but that had changed. He partly attributed that to Jacinda Ardern's elevation to the Labour leadership.

"It was a bit like taking the cork out of a bottle and making it all come out. And that's a pretty tough tide to fight," Dunne told the Herald.

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"I think it's a bit like air out of a balloon...I don't think there is a deep-seated mood for change in the country. But there is a mood for excitement and all that sort of stuff.

"The fact you can have a campaign opening attended by singers and actors, and that's seen as credible, just suggests how superficial this has all become."

Labour's campaign launch at Auckland Town Hall on Sunday featured warm-up routines from singer Hollie Smith and a speech by Don McGlashan.

Dunne is the third leader in just three weeks to resign, following Labour's Andrew Little and the Greens' Metiria Turei.

Dunne's shock decision to quit is a blow for National, which has relied on his vote in Parliament to help achieve a majority. It is also embarrassing, given National has directed its supporters to vote for Dunne, and its candidate Brett Hudson has included this message of his campaign leaflets.

Dunne said he still expected Bill English to be Prime Minister after the election.
"He's got the depth of experience and knowledge and understanding that I don't see matched by anyone else in politics today."

He also backed Hudson to win his seat, and said when he decided to retire it wasn't because he didn't think he could defeat O'Connor.

"I thought to myself...do I go now with a modicum of dignity or end up facing cameras talking about why it all went wrong on election night, if that was the way it was going to turn out."

At times appearing downcast, Dunne, 63, spoke to the Herald in his office overlooking the Beehive and as one of a series of media interviews.

He thanked the people of Ohariu for electing him as a "callow" 30-year-old, and re-electing him 10 times after that, and said it had been a privilege to be their MP.

Dunne said he wouldn't sit around doing nothing in retirement, but hadn't decided what exactly will come next.

"I've had more than half my life here. So it is quite a big shift. Then on the other side of that, I look at my sons who weren't born when I came here, and I'm now a grandparent."

Today, English denied National was now in an awkward position given it had written to voters telling them to vote for Dunne. It was easier to correct "one letter in one electorate" than change billboards across the country as Opposition parties had done in recent weeks, he said.

Hudson said this afternoon that he would now run a full "two ticks" campaign.

"Many people have told me that they have long wanted to vote for their National candidate as well. I am very proud to now be able to seek both their votes."

O'Connor paid tribute to Dunne, saying he had been a "very, very effective local MP" who had been loyal to Ohariu.

"Yes, it's a surprise," O'Connor told the Herald. "But this is an election like no other."

Labour leader Jacinda Ardern acknowledged Dunne's long service as an MP. She said O'Connor was a strong candidate and would continue to work hard: "This race just got very, very interesting. It was already interesting but certainly it will be one to watch."

Act leader David Seymour tweeted Dunne was a "symbol and enabler of how our major parties drift with no real agenda", and had enjoyed ministerial cars and perks when the housing crisis began under Helen Clark and continued under National.

"He has been swept out by the tidal change against do-nothing politics," Seymour wrote.

Green Party leader James Shaw said he had a great deal of respect for Dunne, who had had a long and distinguished career.

"My sense of it is that he could sense there is a change of Government coming up and at this point in his career he didn't want to go into opposition."

Shaw also believed the Greens' decision to stand aside for Labour's Greg O'Connor in the seat played a part, but the final nail in the coffin for the United Future MP was him looking at how the election was going.