Prime Minister Jacinda Ardern has clashed with The AM Show host Duncan Garner after refusing to reveal how she will vote on the question of recreational cannabis legalisation.

In May, the Government announced that New Zealanders will vote in a referendum on legislation to legalise recreational cannabis at the 2020 election.

A simple yes or no question will be presented on the basis of draft legislation that includes a minimum age of 20, regulations and commercial supply controls, limited home-growing options, and a public education programme.

But a Newshub-Reid Research poll revealed on Monday that 48 percent of people disagree with legalisation, while 41.7 percent want it legal.

Ardern wouldn't reveal her position on Tuesday when pushed on it by Garner, saying she wanted to ensure the public was informed and felt she needed to be impartial in presenting that information.

"My vote, ultimately, is only as good as my neighbour's vote. I don't want to be in a position of persuading people either way. I want them to decide for themselves, and I want them to have the information," she told The AM Show.

"For me to have that role, I do think I need to have a level of impartiality in sharing that information and pulling it together."

Two issues she was weighing up the level of access young people have to cannabis and if people should be criminalised for having it.

"I grew up in a town where I saw young people easily access cannabis, and I saw the impact it had on their education, so that worries me.

"At the same time, I don't think people should be criminalised and imprisoned."

But that didn't impress Garner, who said she was the Prime Minister and needed to express her view to the public.

"You are sitting on the fence publicly... you are going to end up voting. You know now how you are going to end up voting," he said.