Speaking in Parliament, in front of a packed public gallery on Wednesday, Seymour implored his fellow MPs to give the bill backing one last time.
"I have listened to New Zealanders talk about their experiences from Kerikeri to Gore," Seymour said.
"Overwhelmingly they have said to me: 'I have seen bad death. If my time comes and I'm not doing well, I want choice. By the way, it's nobody else's business but my own'."
The legislation had passed its second reading 70 votes to 50 and was widely expected to clear the final hurdle.
But neither side was taking votes for granted ahead of the third reading, jostling going on to the wire.
Seymour agreed to major changes to the bill since it was put in the ballot four years ago, in hopes of seeing it succeed.
Most significantly, it now only covers to those as diagnosed as having less than six months left to live, where is at previous included those with grievous and irremediable medical conditions.
Opponents have not been appeased. National's Chris Penk, who has been one of the most ardent critics throughout, on Wednesday night told the House the bill was simply not safe enough to protect the most vulnerable.
"The question is not whether some people will die in the way the bill allows. But whether many people could die in a way that the bill does not allow," Penk said, citing a lack of requirement for witnesses and a cool-down period in the legislation.
He echoed one of the most common criticisms of the bill: that it would see the ill and elderly pressured to take up assisted dying when they didn't need to.
National MP Shane Reti, a doctor, described himself as the "only person in this House who will be commissioned to euthanised New Zealanders".
"I cannot imagine the spectre of euthanasia, ever-present looming over every single consultation … until it is dared to be given light," he said.
His colleague, Whanganui MP Harete Hipango, came dressed in black, during her speech describing the legislation as a deeply flawed "kill bill".
Hutt South MP Chris Bishop, meanwhile, described the status quo as "fundamentally intolerable to a civilised and humane society".
"I do not accept the argument; that has been put by some; that painful death is just something we should blindly accept," Bishop said.
Seymour, for his part, defended the bill as having enough safeguards against coercion – it requires doctors to stop the euthanasia process if they suspect pressure – a tried to ease concerns it would affected anyone but the dying.
He, among many others, also paid tribute to Wellington lawyer Lecretia Seales, who inspired Seymour's initial efforts and revived public debate about euthanasia.
Seales died in 2015, a day after the High Court rejected her bid to be allowed to access assisted dying.
"Who else but me should have the authority to decide if and when the disease and its effects are so intolerable that I would prefer to die?" Seymour said, quoting Seales.
Her mother, Shirley Seales, was among those in the public gallery watching the vote.
The bill is the fourth to try to legalise assisted dying – starting with one in 1995 – and the only one to clear even a first reading.
The other significant change made to secure its passing was the addition of the requirement of a referendum.
It was a demand of New Zealand First – which threatened to vote against the legislation, likely shutting it down if it didn't get a plebiscite – and accepted begrudgingly by MPs who were determined to see the bill pass.
NZ First MP Tracey Martin on Wednesday defended asking the public to decide.
"New Zealanders elect us. But they do not elect our consciences," she said.
Finance Minister Grant Robertson, voting in favour of the bill, lamented that Parliament could not have made the decision itself.
Since it was introduced, the bill has been met with constant, vocal opposition. The vast bulk of submissions to its lengthy Select Committee process were against it, and even on Wednesday a rally was being held on Parliament's lawn.
MPs in opposition unsuccessfully tried put up more than 100 amendments to the legislation during a series of debates in the House.
While campaigning on the referendum will next year likely be even more heated, historical polling so far has suggested it is likely to pass.
However, the referendum question will ask voters whether they support the End of Life Choice Bill becoming law, rather than assisted dying, and the effect that will have is still unclear.
A poll in July found there was 72 per cent backing for some kind of assisted dying for the terminally ill among the public. Support over the past 20 years has averaged to about 68 per cent.
A brief history of euthanasia legislation in New Zealand
• 1995 – Death with Dignity Bill sponsored by then National MP Michael Laws failed at its introduction, 29 votes to 61.
• 2003 – Death with Dignity Bill sponsored by New Zealand First MP Peter Brown failed at its first reading, 58 votes to 60 and one abstention.
• 2012 – End of Life Choice Bill sponsored by Labour MP Maryan Street was put into the members' ballot but was withdrawn in 2013 at the request of then leader Andrew Little.
• 2015 June – Petition of Maryan Street and 8974 others presented to Parliament calling for investigation into medically-assisted dying for the terminally ill.
• 2015, Oct – End of Life Choice Bill sponsored by Act leader David Seymour was put into the members' ballot.
• 2016 August – 2017 August - Health committee inquiry into assisted dying.
• 2017 June – David Seymour's End of Life Choice Bill drawn from ballot and introduced.
• 2017 Dec - Seymour Bill reinstated from previous Parliament and passed first reading. 76 to 44.
• 2018 – 2019 Seymour Bill select committee hearings.
• 2019 June – Seymour Bill passes Second Reading, 70 to 50.
• 2019 Oct – NZ First amendment requiring law to be triggered only by referendum at 2020 election passed in committee stage, 63 to 57.
• 2019 Nov – End of Life Choice Bill passes its final reading.
• 2020 - Referendum to be held on the bill alongside the general election
