New Zealand First MPs will vote for a bill to legalise voluntary euthanasia at the first stage – after David Seymour agreed to support a binding public referendum on the issue.

The Herald can reveal that last-minute talks between the Act leader and NZ First today have resulted in Seymour agreeing to support NZ First's push to make any law change conditional on a public referendum.

MPs are set to vote tonight on Seymour's End of Life Choice Bill tonight. It is a conscience vote and MPs range from Prime Minister Jacinda Ardern's support to National MP Maggie Barry's description of it as a "licence to kill".

Most parties are allowing MPs to lodge conscience votes, but NZ First's nine MPs will vote as a bloc.

Advertisement

The party's coalition agreement with Labour included an agreement to "allow a conscience vote for MPs on New Zealand First's Supplementary Order Paper to the End of Life Choice Bill, which provides for a referendum".

NZ First MP and party whip Clayton Mitchell told the Herald if the legislation passes its first reading tonight, his party expected that Supplementary Order Paper to be introduced either in the select committee stage, or after the second reading during the Committee of the whole House stage, where amendments can be proposed and voted on.

Mitchell said the amendment would mean that if the End of Life Choice bill passed its final reading, it would not come into effect until a majority of the public supported it in a binding referendum.

He said the party's position had been to vote against the legislation tonight, unless Seymour agreed to actively support a binding referendum on the issue.

Seymour had today told him he would support NZ First's Supplementary Order Paper in the select committee stage, and mention that support in his speech tonight.

If the order paper goes through, NZ First MPs will continue to support the legislation as one bloc, Mitchell said - meaning MPs like Shane Jones who oppose euthanasia will vote in support of the legislation.

"We are the only party that is 100 per cent united on this belief that it shouldn't be up to us to make that decision. And therefore we won't."

Seymour confirmed he would support such an amendment if one was put up at a later stage.

Advertisement

He expected the first reading to be tonight: "I'm feeling good about it".

Mitchell said he understood it would be up to Labour's MPs to individually decide whether to support the NZ First Supplementary Order Paper.

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.