Politicians fiercely opposed to a bill legalising assisted dying say they'll be putting forward "amendments on amendments" as a prolonged and messy battle begins in Parliament.

Act Leader David Seymour's End of Life Choice Bill is set to return to the House today and MPs are set to debate a raft of changes proposed since it passed its second reading, in June, 70 votes to 50.

Seymour is expecting the committee of the whole house stage to take about four months, but it could stretch well into next year if critics get their way.

Hours before the debate was due to begin on Wednesday, a group of staunch National Party opponents - led by Maggie Barry, Chris Penk, Simon O'Connor and Alfred Ngaro - laid out their strategy.

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They've previously promised to lodge 120 amendments for debate.

"I would imagine it could quite easily go beyond that," Barry said today.

"Because you can make amendments on amendments."

But the group deny they're just stalling.

"It is not filibustering. Filibustering is where you put in useless time-wasting things. This is genuinely reflecting the genuine concerns of the vulnerable," Barry said.

"For the thousands of people who came before the select committee their issues have not yet been debated by the whole of the house."

Act Party leader David Seymour questions whether opponents of his bill will be able to stall it for long. Photo / Mark Mitchell
Act Party leader David Seymour questions whether opponents of his bill will be able to stall it for long. Photo / Mark Mitchell

The justice select committee heard a record 39,000 submissions, but was unable to come up with major changes to the legislation and Barry, its deputy chair, in April described it as unworkable.

Among the many changes the opponents now want to make are: increasing the age of eligibility for assisted dying from 18 to 25, tightening competency requirements and including more references towards cultural values.

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Meanwhile, Seymour has proposed his own list of changes in a bid to secure some much-needed votes.

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

Seymour's most significant proposal removes the "grievous and irremediable" part, limiting the bill to just those with six months live, in order to win the Green Party's vote.

Its members were concerned about implications the original wording could have on the vulnerable groups, such as the disabled community.

On Wednesday, the Greens said they would back the amendment.

"The Green Party would not have been able to support legislation that extended medically-assisted dying to people who aren't terminally ill," co-leader Marama Davidson said.

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"We are pleased to see proposed amendments that resolve this".

Barry, meanwhile, said the change was meaningless.

"It doesn't make any difference, because no one understands it," she said.

"This phrase [grievous and irremediable] makes no sense. It has no case law, it has no widespread understand in New Zealand. It was as loose as a goose."

Seymour says he is taking comfort in a few strategic advantages.

Parliament's rules mean his amendments will be heard first and changes cannot contradict each other, meaning plenty could be ruled out of order.

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"In practice we know those who are opposed to the bill are going to do everything they can to prolong the debate," Seymour said.

"But there is a limit to what they can do because there's not as many of them as people think."

However, the biggest obstacle to his bill may come from NZ First.

Its MPs are demanding a public referendum be a requirement of the legislation and last week put forward a supplementary order paper to that effect.

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

Seymour has been trying to rally support for a referendum but it is not yet clear if he has the numbers.

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The plebiscite will be among the last changes debated, meaning a large number of MPs are waiting to see the final bill before deciding.

None of the National MPs fronting media on Wednesday would put their backing behind the version of the referendum NZ First had put forward.

Without NZ First's nine votes, Seymour cannot afford to lose even a single net "yes" from the second reading.

The first debate in the stage will begin on Wednesday evening.