New Zealanders may get the opportunity to vote on legalising cannabis and voluntary euthanasia at the same time, possibly as early as next year.

Labour agreed to hold a public referendum on legalising cannabis for personal use at or before the 2020 general election.

The Government now says it could be held ahead of the election to make sure it did not overshadow the election campaign.

Parliament is also considering a bill to legalise voluntary euthanasia, and NZ First has said its support is conditional on a binding referendum on the law change.

The bill's sponsor, Act Party leader David Seymour, has agreed to that proposal, though it will still need majority support in Parliament to proceed.

Justice Minister Andrew Little said there could be some benefits to holding two referenda at the same time.

"If you're going to do one, you might as well do a job lot," he said.

"It would make sense to not have to spend a lot of money on a succession of referenda."

There could also be some value in holding a referendum outside the election period, Little said.

Cabinet had not yet considered the cannabis referendum, but when it did the date would be one of its considerations.

"Do we run it at the same time as the general election, as there will be some cost saving in that?" Little said.

"The other question is do we want the general election dominated by the referendum which might suggest an earlier timetable than 2020."

The earliest a referendum could take place would be late 2019, because it would need to be preceded by a public information campaign.

Holding the referendum outside the election period could open up the Government to criticism over the cost.

Labour was critical of the $22m cost of the National-led Government's public referendum on changing New Zealand's flag in 2015-16, though that included the costs of the design process, expert panel, and nationwide roadshows.

Green Party co-leader James Shaw said it made more sense to hold a referendum at the same time as an election.

"Just on the basis of cost and convenience – people are going to be going to the polling booths anyway. And the mail is a declining way of staying in touch in this country so I don't know what the turnout of a postal referendum would be."

He did not share Little's concern that a referendum could detract from a general election.

"I think people would treat it as a distinct issue," he said.

NZ First leader Winston Peters said today that he wanted the cannabis referendum to be binding on the Government.

But Little said neither of the referenda were likely to be binding: "We don't typically do binding referenda in this country."

Labour has previously said it did not want a euthanasia debate in election year.

Former Labour leader David Shearer went as far as to make former Labour MP Maryan Street remove her pro-euthanasia bill from the private member's ballot in 2013 in case it was pulled in election year.