Ardern said that the Government did not have a position on whether cannabis should be legal, and that was why the public would decide.
She said for that reason she was inclined not to reveal how she would vote.
The referendum is part of the Labour-Greens confidence and supply agreement.
Officials are currently putting together a draft bill that will outline a proposed regulatory regime for legal cannabis, which will then be put to the public in a vote at the 2020 election.
According to both new polls, Green voters were the most likely to vote "yes" in next year's referendum, while National Party voters were most likely to vote "no".
Last month the Government revealed details about next year's referendum.
There will be a simple Yes/No question at the referendum, asking voters to either favour the regulatory regime outlined in the draft bill, or the status quo.
Details already released of what will be in the bill include:
• Allowing products to be bought only in licensed premises from a licensed and registered retailer, and banning online or remote sales
• A ban on using cannabis publicly, allowing it only in special licensed premises or on private property
• Controls on the potency of cannabis in available products
• A legal purchase age will be 20
• A ban on advertising of cannabis products, and requiring products to carry health information
• A state licensing regime to control the supply chain and the manufacture of all products, such as resins and edibles
• A ban on all imports of cannabis unless through a state-licensed wholesaler
Other details still being worked out include the limit of potency, the rules of the licensing regime, the level of taxation and how much of that should fund health and addiction services, and whether cannabis-related convictions should disqualify a person from working in a legal market.
Whether the vote will be binding has been questioned, as the Government could choose to ignore the referendum results, or change the bill after the vote.
The National Party has not committed to following through with the will of the people until at least after it can see the draft bill.