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The polarising legislation - which would let terminally ill adults request voluntary euthanasia - now has to pass a third reading in the House in November. If it does - which is likely - the public will have the last word next year.
The referendum would sit alongside a separate one on legalising recreational cannabis.
During a debate on Wednesday over whether to hold a euthanasia plebiscite, New Zealand First's members made clear they would vote against the legislation as a block if the referendum was rejected.
NZ First MP Mark Patterson took an entire speech to make the point.
The End of Life Choice bill passed its second reading - in June - 70 votes to 50 and without NZ First's nine votes its odds of surviving a third vote were miniscule.
That left many politicians who support the legislation begrudgingly voting for a referendum to keep the bill alive.
Labour's Kieran McAnulty reflected on the dilemma in his speech to the House.
"In a perfect world I would have voted no to a referendum … The MPs of this house have been given a job" McAnulty said.
"But I am a bookmaker and I know how to count."
Seymour himself said he didn't feel strongly either way about the referendum, but saw it as a necessity. He has been working with NZ First to try to get support.
Meanwhile, opponents of the legislation railed against the referendum on Wednesday, saying it would be an abdication of power by MPs paid to make hard decisions.
National's Alfred Ngaro told the House it would be "irresponsible".
"Why would you want to unleash a complex and difficult and socially-impacting decision onto a public by a 'yes' or a 'no'?" he asked.
However, Labour MP Louisa Wall – who strongly supports the legislation – also spoke against a referendum, saying the public debate it would cause would be too divisive and that she feared many voters did not know enough about the issue.
NZ First's Jenny Marcroft – the sponsor of the referendum – said politicians needed to trust the public.
"This issue basically, directly affects the fabric of society and so we believe that temporarily empowered politicians … we alone should not decide on the bill," Marcroft said.
The referendum question would ask the public whether they support the End of Life Choice Bill becoming law, rather than a more general question about euthanasia.
Seymour has made a large number of changes to the legislation since its second reading to make sure he retains supports for the upcoming third.
Most significantly, it now only gives access to assisted dying to those with six months left to live, while it previously also covered those with "grievous and irremediable" medical conditions.
It's not yet clear whether the changes have won any additional votes or kept all those from the second reading. It needs 61 votes to pass, making NZ First's backing crucial. With it's support, it will likely clear the third reading comfortably.
The legislation will return to Parliament for its final reading on November 13.
Polling has suggested the public would vote for euthanasia at a referendum.
Victoria University research fellow Jessica Young, an expert on the issue of assisted dying, said over the past 20 years public support for some form of assisted dying has averaged about 68%.
Public campaigns surrounding the End of Life Choice Bill have not shifted the mood much either, with a 1 News Colmar Brunton Poll in July – ahead of the second reading – saying about 72% of the public supported assisted dying of some sort.