Seymour said he was delighted Parliament would debate voluntary euthanasia. He turned down a ministerial portfolio to focus on the issue.
"This is morally, democratically and legally the right thing for Parliament to do," he told reporters at a press conference this afternoon.
While he believed some MPs may try to filibuster the legislation, he believed it could pass later this year under the next government.
"I think we will easily pass this legislation. Its time has come."
Prime Minister Bill English, a practising Catholic, has previously said he is firmly against legalising euthanasia. Leader of the Opposition Andrew Little has said he would support legalisation if it had appropriate safeguards.
Deputy Prime Minister Paula Bennett said today she was undecided on her vote, and she wanted to take some time to consider her position.
"I'm not an absolute no. So I suppose I want to see what the consequences are, what the kind of detail is."
A number of National MPs said they would vote against the bill today, including Foreign Affairs Minister Gerry Brownlee.
National and Whanganui MP Chester Borrows said he was firmly against it.
"We have a horrific record on suicide and I think it sends a message that sometimes it is okay to top yourself. And I disagree with that."
Economic Development Minister Simon Bridges said he was likely to vote against it, but may vote for it to go to a select committee to it could be properly debated.
"Ultimately life is sacred," Bridges said. "And I think there are ... 'thin edge of the wedge' arguments that concern me."
New Zealand First MPs said they wanted a referendum on the issue. If that could not be achieved, they would likely vote against the bill.
The Green Party has a formal policy of legalising euthanasia for terminal patients.
Health spokeswoman Julie Anne Genter said Seymour's bill went further, and her caucus would need to discuss it before deciding its vote.
Maori Party co-leader Marama Fox said the bill was "not something we would support at the moment".
"I've sat holding the hands of dying people over and over again ... so I have a good understanding of this and I've never once heard one of those people tell me they want to go early.
She supported the debate, but said it was not something she personally supported: "It's a conversation the nation is ready to have."
POLARISING ISSUE
The controversial bill represents the best chance for voluntary euthanasia to be legalised in New Zealand - although the issue is deeply polarising and many MPs including Prime Minister Bill English are firmly opposed.
Despite opinion polls showing strong public support for legalising euthanasia, the issue's extreme divisiveness has meant politicians have been reluctant to champion a change.
That has all changed now Seymour's bill has been drawn from the lottery of a ballot.
His The End of Life Choice Bill is based on an earlier piece of legislation drafted by former Labour MP Maryan Street.
It would allow mentally competent New Zealand adults who have a terminal illness likely to end their life within six months, or have a grievous and irremediable medical condition, the choice to ask a doctor to help end their life at the time of their choosing.
The Director-General of Health would establish a group of medical practitioners who would maintain a register of health professionals willing to participate in assisted dying.
A new process would require two medical practitioners to be satisfied a person meets the required criteria. The second would be independent of the patient and initial doctor.
Seymour has expressed confidence that his bill will pass a first reading in a conscience vote.
That confidence is based on conversations with MPs in Parliament's corridors, researching public statements, and pressure from voters (Seymour polled Epsom before putting up his bill and 69 per cent were in favour).
A parliamentary inquiry into voluntary euthanasia began last year.
But that is unlikely to lead to a recommendation legislation be introduced and Seymour said he did not want to wait for the outcome of the inquiry.
Both Labour and National have ruled out making euthanasia a priority if they are in power after the election.
The inquiry was prompted by a petition which followed the death of Wellington lawyer Lecretia Seales, who unsuccessfully sought a High Court ruling that would have allowed her doctor to help her die without criminal prosecution.
Seales' husband Matt Vickers said today she would be "over the moon" that Parliament would now debate the issue.
Months before she died in mid-2015, Seales unsuccessfully appealed before the High Court to be able to choose a medically assisted death.
