The question of euthanasia is set to be thrown to Kiwis after New Zealand's parliamentarians failed to reach a majority on the issue despite a four-year parliamentary battle and record public consultation.

Granted a conscience vote, MPs voted 63-57 to put the matter to a referendum after NZ First threatened to pull its support without a public poll.

In heated debate on Wednesday, the ultimatum from the minority partner in Jacinda Ardern's coalition government won the day.

The party of Deputy Prime Minister Winston Peters pledged its nine MPs would only support the bill if it was put in a referendum alongside the 2020 election.

And in a tight evening vote, supporters of the End of Life Choice Bill surveyed the numbers in the 120-strong chamber, and swung their support behind a referendum as the only tenable way of seeing euthanasia legalised.

The bill is not yet law and therefore the referendum is not yet set.

The bill must still pass a final vote but that is considered likely after the most contentious amendments cleared the house.

The bill was first tabled in 2015 by libertarian MP David Seymour, the parliament's sole ACT member.

It was brought on in 2017 after Ardern took power and brought on one of the most substantial consultations for a piece of legislation in New Zealand's history.

More than 39,000 submissions were received, with public hearings in 14 cities allowing about 1350 people and organisations to give oral submissions.

On Wednesday night, the debate had passionate supporters on both sides - as well as some that were genuinely torn.

"I have never been so confused by a bill in my life," Labour MP Willie Jackson said, before voting against.

"It's the toughest vote that I've ever had."

National MP Harete Hipango made a fire-and-brimstone contribution, saying "this is a bill that seeks to kill people".

"I seek to kill this bill because of the repugnancy of what it seeks to do," she said.

Ardern voted in favour of the bill, but registered her concerns with putting such a contentious matter to a referendum rather than a parliamentary vote.

If public support is any guide, euthanasia is likely to become law after the election.

Proponents of the bill cite public polling north of 70 per cent in New Zealand for euthanasia.

Ardern's government has already agreed to put the legalisation of cannabis for personal use to a referendum alongside the election, which is expected in September or October.

Australian Associated Press