And he has defended National leader Judith Collins, telling Newstalk ZB's Mike Hosking today that she was doing "really really well" despite hiccups on the campaign trail.
On the cannabis referendum, Key said he would be voting no.
"The point of a referendum, right, is to give people a chance to have their say.
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"Whether you want to vote yes or no for cannabis, basically, it doesn't really matter whether Jacinda Ardern is voting one way or the other - you'll make up your own mind.
"If you want to see more drugs in New Zealand society, vote yes. But if you don't, vote no."
Key said it was a "load of nonsense" that the Government would make more money - it would instead be spending more on mental health issues, for example, he said.
"The fact that the gangs won't be there - what a load of junk.
"Of course the gangs are gonna be there. They're going to actually have a cheaper product because they don't pay taxes.
"In the end, if you want a society where there are more prevalent drugs and your kids are more likely to take them, vote yes. Personally, I'll be voting no."
The 'yes' campaign has not argued that legalising would see the black market completely displaced, but that it would reduce cannabis-related harm.
Key defended National's election campaign. He said that some of the criticism Collins had faced was over small issues that did not have an impact on the electorate.
"I don't think too many voters are going to hold that against Judith," Key said, addressing claims of a rent-a-crowd of National supporters at Collins' walkabout in Ponsonby yesterday.
Key said every party - not just National - had supporters who always knew where the leadership was on any given day.
He said that sort of issue highlighted in the media was more of a "process issue" that blocked the main messages being put out by a certain party.
Key acknowledged that Ardern's success or 'it' factor may have also had a lot to do with her being at the forefront during the Covid-19 response.
"That just does add to a celebrity factor," he said.
"The principle is, it is fundamentally easier when you are Prime Minister."
Questions about Collins' leadership were raised when she promised an Auckland Council review, if National was elected, live on Newstalk ZB earlier this week.
National's council spokeswoman, Denise Lee, was not aware of that and sent an email that described Collins' announcement as a "highly problematic idea". The email was later leaked to media.
The National leader also came under fire yesterday when a walkabout with media in Auckland's Ponsonby saw them bump into a crowd of National supporters.
Last weekend, Collins was pictured offering a prayer at St Thomas Church in Tāmaki while out to vote with her husband David Wong-Tung.
Images of her kneeling at a pew resulted in claims of the leader seemingly politicising her faith - something she later refuted as she had not invited media to photograph her at that moment.
Collins has said she would remain in the role of leader if her party was unsuccessful come next Saturday.
