People believe Jacinda Ardern kept them safe - Sir John Key on why he thinks Prime Minister is so well-liked

Sir John Key has come out with some theories as to why so many more people voted for Prime Minister Jacinda Ardern and Labour at last month's election.

Speaking about National's poor election result on The AM Show, the former Prime Minister and National leader said it was always going to be tough to attract enough voters to win 2020.

"They did it [voted Labour] for three reasons actually; some of them will say, 'I like Jacinda' - the younger audience particularly," Sir John said on Monday.

"Then there'll be an older audience who genuinely said, 'she kept me safe.'"

Ardern and her Government has been globally praised for its response to the COVID-19 pandemic, something political pundits have said was a big reason why she and Labour gained so much support at the election. 

But Sir John believes Labour collected the biggest group of votes from those who wanted Labour to garner enough support to govern alone without the Green Party. In the lead-up to the election, National was warning voters Labour would introduce a wealth tax, despite it being a Green policy and Ardern ruling it out on several occasions.  

"When we [National] started talking about the wealth tax, they [voters] thought, 'I don't want a wealth tax', and so the only way to have that control mechanism is to make sure that Labour can govern alone," Sir John said.

"My view on all of that is, again, to get them back we've got to get back on our agenda.

"The simple message for The National Party is we've got a mountain to climb; we've got to get back 413,800 voters - that's an awful lot of people.

"If you're going to do that then you've actually got to do the right things, which is; have messages that are about the voters, that hold the Government to account, and that contrast your policies with their ones."

But Sir John believes Ardern is beatable and told National's AGM in Wellington on Saturday the party's MPs had to stop leaking as they were before the election. 

"I was in a unique position to deliver that message - there's some validity in that this was a really difficult election to win - [but] if we want to win we actually have to win back those voters," Sir John told The AM Show. 

"We have to get voters who voted for us only three years ago to say - 'they're on the right message, they're thinking about me, they've got better policies, they're exposing the weaknesses of the Government'.

"Unless you start doing that and get control of that agenda, you can't win because you've got to win those votes, you can't just hope they'll come back to you."