A long battle is set to break out in Parliament as politicians begin to hammer out the details of a controversial bill legalising euthanasia.

David Seymour's End of Life Choice Bill passed by 70 votes to 50 at its second reading last month, but could be left hanging by a thread as a complex and drawn-out Committee of the Whole House begins on Wednesday.

This stage will see politicians debating and individually voting on possibly more than a hundred changes to the bill, in a process that may stretch - optimistically - four months.

The legislation originally allowed the terminally ill or those with "grievous and irremediable" conditions to legally request assisted dying, with clearance from two doctors.

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Facing concerns the "irremediable" clause could have wide-reaching implications for groups such as the disabled community, Seymour this week put forward an amendment, among a series of others, to limit access to just those with six months to live.

But it's another proposed amendment that could put a spanner in the works.

NZ First's members are demanding a public referendum be added to the legislation and last week put forward a supplementary order paper to that effect.

They will vote "no" on Seymour's bill if the referendum isn't accepted by the House.

Act Leader David Seymour has limited his bill to just those that are dying in hopes of securing backing. Photo / Paul Taylor
Act Leader David Seymour has limited his bill to just those that are dying in hopes of securing backing. Photo / Paul Taylor

"If we don't get the plebiscite option, then we don't believe 120 MPs temporarily ensconced here have the right to change the law for euthanasia purposes," NZ First's Shane Jones said.

Without the party's nine votes, Seymour cannot afford to lose even a single net aye from the second reading.

He has promised to support the referendum, and has been raising it with prospective "yes" voters while lobbying and negotiating in recent weeks.

He faces a sea of maybes on the issue.

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The referendum amendment will be among the last to be debated, meaning plenty of MPs are taking a wait-and-see approach.

"It's fair to say that other people around Parliament have some reservations and are weighing up whether they'll support it to see the bill go through," Seymour said.

Some firm supporters of the bill overall, such as Labour's Louisa Wall, are totally opposed to a referendum on the grounds the following debate could cause more harm to the vulnerable and would be an abdication of responsibility.

NZ First's Shane Jones says it's either a referendum or a
NZ First's Shane Jones says it's either a referendum or a "no" vote from his party. Photo / John Stone

Others, such as Justice Minister Andrew Little, say they'll vote for it if it's necessary to make the law happen. Prime Minister Jacinda Ardern says she doesn't like the idea of a referendum, but has stopped short of ruling it out completely.

If a plebiscite does happen, it may have good odds of passing.

A 1 News Colmar Brunton Poll this week found 72 per cent of those surveyed believed "a person who is terminally or incurably ill should be able to request the assistance of a doctor to end their life".

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Aside from the referendum, a number of MPs, such as Labour's Willie Jackson, gave their backing at the second reading on the grounds the bill would be tightened up and include extra safeguards.

The Select Committee that looked at the bill failed to come up with substantial fixes and Seymour now hopes the list of amendments he's put out this week will do the trick and swing a new few votes his way.

Other changes include requiring the patient to always initiate the discussion about assisted dying.

Wall and National's Lawrence Yule are also putting forward their own amendment to let the Family Court make the final decision.

Meanwhile, Chris Penk will be part of a group of National MPs who are opposed and have promised to introduce more than 100 amendments during the committee process, potentially dragging it into next year.

Focus: PM Jacinda Ardern says she does not personally think a referendum is needed for the End of Life Choice Bill Video / Mark Mitchell

They deny it's stonewalling, and say they want to fix and point out flaws.

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National's Judith Collins - who swapped to a "yes" at the second reading - on Tuesday said she hoped no one would be playing "silly buggers" with the process.

She wouldn't specify who she meant.

Seymour takes solace in a parliamentary rule that means amendments cannot contradict each other, and says he has a strategic advantage by having his proposed changes heard first.

The amendments will be debated on Members' Days at Parliament, which only take place once every second week, and each of its four parts has to be considered in a separate session.

Once all parts have been voted on, the bill will go to a third and final reading.