Opponents of euthanasia are warning that politicians have not taken their concerns into account after voting through David Seymour's bill.
Lawyers, doctors and mental health workers have spoken up, believing they can still convince a majority of voters to say "No" to voluntary euthanasia at the referendum in 2020.
But Mr Seymour said they are scaremongering and all the facts are on his side of the debate.
Mr Seymour said it has been four years of long suffering and he is thankful to everyone who has helped.
But opponents are dismayed. Dr Sinead Donnelly said this week her online letter attracted its 1500th signature – 1500 doctors who say no to euthanasia.
"Reaching that target at this time just reflects the deep concern of doctors about the role this bill implicates them in, and the impact it will have on the practice of medicine.
"This bill actually puts a knife through the heart of medicine."
Dion Howard is a registered nurse working in mental health and said 21 mental health workers have penned a letter to MPs ahead of Wednesday night's vote, warning of what they say are links between euthanasia and suicide.
He said he has first-hand experience of his young clients using the same reasons for their suicidal inclinations as euthanasia advocates.
"They'd use things like bodily autonomy and unbearable suffering, and quite articulately as well. So when you're working with someone who has chronic suicidality you factor in every possible risk that may shift that needle for them."
Mr Howard said the government has a duty of care for the vulnerable and Mr Seymour has not proven that the regime of euthanasia in the law is safe.
"Our Prime Minister has said one suicide is one too many. Yet we have a trending suicide rate that's the worst it has been since the 1990s. And yet we're so readily dismissing a duty of care.
"The onus of proof rests on the proponents of the bill – not the opponents – to say that it is safe."
But Mr Seymour said the debate has been rigorous, and the claims that an increased suicide rate relates to euthanasia are not rigorous.
"They are now using terms like 'suggestive evidence'. That is not an academic term," Mr Seymour said.
"That is not evidence, that is speculation masquerading as evidence. Just because somebody has some expertise in a field, doesn't mean they have expertise in another field, such as analysing the actual connection between statistics."
Lawyers for Vulnerable New Zealanders spokesperson Richard McLeod said their concerns around coercion weren't dealt with in the committee of the whole house, despite many supplementary order papers. He said many people could be taken advantage of.
"Coercion and pressure in our vulnerable communities is a really serious and growing problem.
"We've got the widespread coercive forces of elder abuse, we've got the coercive forces of loneliness and marginalisation, especially amongst our elderly.
"We've got the coercive force of not wanting to be a burden, and of course we've got the coercive force particularly on Māori, of a healthcare system which is increasingly failing them.
"These profound coercive forces already at work need to be checked, and this bill fails dismally in checking them."
But Mr Seymour said there's no evidence for that.
"Actually the people who avail themselves of assisted dying are people who have the assertiveness in their character, the education, the resources to navigate a fairly arduous system of bureaucratic checks and balances to become eligible."
Dr Sinead Donnelly said she was in the chamber of Parliament on Wednesday when MPs were debating the third reading of the End of Life Choice Bill. She said she is sad that medical professionals have not been respected.
"I'm really just dismayed that people, who are lay people even though they're MPs, would speak in such a disrespectful way about a group of people who dedicate their life in the service of people who are vulnerable and sick."
Mr Seymour said that he and the 68 other MPs voting in favour, are on the right side of history. He said there are 150 million people living in jurisdictions where assisted dying is legal, though The Lawyers for Vulnerable New Zealanders say that leaves billions of others around the world who don't.
There are more than 3.5 million eligible voters in New Zealand who will have to decide which box to tick at next year's referendum – that is likely fewer than 12 months for those sitting on the fence to be convinced one way or the other.