Just about everything went wrong with the National Party's campaign from rolling Simon Bridges, insulting fat people and the a full-blown assault on the Greens' wealth tax, says an experienced campaigner.

Former Labour Party president Mike Williams said the campaign was a disaster.

"I have never seen a worse campaign run by a political party and I've been involved in 40 elections in Australia and New Zealand."

National suffered a devastating defeat in the election on Saturday, losing 20 MPs by securing just 26.8 per cent of the vote and winning 26 electorate seats. It was its worst defeat since 2002.

Leader Judith Collins likened her task as taking over the leadership as a "hospital pass" while her deputy Gerry Brownlee - who lost his Ilam seat - said National had a "shocker of a year".

Collins said the party always reviewed its campaigns after elections and said this year's one would be "very thorough" and the terms of the review will be decided by the party's board.

Party president Peter Goodfellow, who's been in the job since 2009, is up for re-election next month but Williams said it was time for him to go and be replaced with someone new.

National was always going to have an uphill battle up against an incredibly popular Prime Minister but it then made a lot of bad decisions, said Williams.

For starters, rolling Bridges was a mistake and the result of poll-driven panic after adulation was heaped on Jacinda Ardern for leading the country through the Covid-19 response and lockdown.

National should have kept their nerve but then they chose the wrong leader by picking Todd Muller who quickly stood down because of his mental health.

But the caucus should have recognised he wasn't suited for the role.

"Instability in political parties is the smell of death to a lot of voters."

"Then they did some really silly things - two examples in the last week was attacking fat people which is thirty per cent of the population. I've never seen anything like that. You've got to be just loopy."

The week of the election, Collins caused outrage by describing obesity as a weakness and saying people should not "blame systems for personal choices" after being asked about it on the campaign trail.

As well, Williams said the continued attacks on the Green Party's wealth tax "drove some soft National people to vote Labour".

That would explain why so many unexpected deep blue seats flipped to red, like Ilam and Taranaki-King Country, said Williams.

He pointed to centre-right Mediaworks personality Mark Richardson saying he was considering voting Labour to keep the Greens away from the Cabinet table.

And there appeared to be a lack of enthusiasm on the ground when it was clear they weren't going to win - but they could have lessened the loss, Williams said.

Former campaign chair Paula Bennett, who stood down at the election after being ousted as deputy alongside Bridges, also said National was disorganised and didn't have a clear plan.

"I think that people had a reason to vote for Labour and that was the Prime Minister's popularity, they got us through Covid, they only had three years and they were very good at articulating that and I'm not sure that National articulated well and clearly enough the alternative."

She told TVNZ's Breakfast she'd been working on a strategy for about 12 months before it "went out the door" when the job was taken off her but refused to say whether she thought the party would have done better under her and Bridges.

Bennett said the change of leadership "was the start of the end".

"There's no two ways about it."

She commended Collins for the job she did and said the caucus should keep her as leader.

"Right now is the time for National to take a damn good look at itself and I think that picking off individuals and trying to do blame that way is not going to be helpful for them at all."

National Party whip Matt Doocey said the result could be traced all the way back to the ill-discipline of Jami-Lee Ross who leaked Bridges' travel expenses, then called him corrupt before standing down and being expelled on the same day.

They then went through the scandals of Clutha-Southland MP Hamish Walker leaking Covid-19 patient details in June, then Rangitata MP Andrew Falloon resigning in July over sending explicit images to young women.

As well the party went through two leadership changes in quick succession.

"If you don't look like you can manage yourself, no one's going to trust you managing the country," Doocey said.

Doocey said there was a culture of leaking in the party which would be addressed in the review.

They would then need to reset then focus, he said.

"Because this job is about the people we serve in the country - it's not about us."