After months of stonewalling, Prime Minister Jacinda Ardern has revealed she voted 'Yes' in New Zealand's cannabis referendum on the same day preliminary results suggest the proposal will narrowly fail.

Ms Ardern kept her position private through the campaign, saying she wanted her caucus and everyday Kiwis to vote in the "reeferendum" without her influence.

A spokeswoman for Ms Ardern revealed her tightly guarded vote 10 minutes after the release of preliminary results showed 46 per cent in favour and 53 against.

Given the Labour leader's historic popularity - evidenced in her landslide re-election - drug law reform advocates were bereft to learn a would-be supporter sat the debate out.

NZ Drug Foundation board member and AUT academic Khylee Quince said she was not angry at the prime minister, "just so disappointed".

"(Her public support) would have been quite significant," she told AAP.

"People see her as thoughtful and empathetic, the leader of the nation and she pushes buttons of particular demographics.

"For the 20 to 25 per cent out there of undecided Pakeha (non-Maori), she's their lady.

"Of course, it may not have politically been a wise move to support it ... she's quite risk averse and I respect her right to privacy."

On Friday, the Electoral Commission published the preliminary results of two referendums held alongside the October 17 poll.

The End Of Life Choice ballot will pass overwhelmingly, as predicted, with 65 per cent of Kiwis voting in favour, and 34 against.

The cannabis legalisation question was much tighter, with 46.1 per cent voting 'yes' and 53.1 per cent 'no'.

The results aren't set in stone.

There are around 500,000 "special votes" still to be counted, which are typically weighted towards progressive voters.

The Yes vote trails No by around 167,000 votes.

Justice Minister Andrew Little said that margin gave virtually no prospect of the result changing.

Greens MP Chloe Swarbrick, who won the Auckland Central seat while campaigning for cannabis legalisation, took a swipe at the PM when asked if her support could have made the difference.

"I'm in the Greens because I have courage in my convictions," she said in Auckland.

"There's every chance that this does flip when it comes to the specials but it will be that razor thin margin."

Ms Ardern bristled at suggestions in the lead-up to the poll that her secrecy on her vote was cowardly, or designed not to scare off conservative voters who were considering voting for Labour.

Leading anti-reform group Say Nope To Dope spokesman Aaron Ironside admitted Ms Ardern's fence-sitting was likely the difference.

"There's no doubt she would have swayed some ... but ultimately when people thought about the legalisation of recreational cannabis, they've made the right decision," he told AAP.

"We're glad the PM left it in the hands of the people. That's the point of a referendum.

"We're pleased. It's a victory for Kiwi commonsense."

Australian Associated Press