Sir Bill English has returned to Parliament on Thursday to argue against proposed euthanasia legislation.
The former Prime Minister was joined by his wife Dr Mary English to submit against against David Seymour's contentious End of Life Choices Bill.
"In my view even the supporters of euthanasia should oppose this Bill, it relies on safeguards, the safeguards will not work," Mr English told the Justice select committee.
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Mr Seymour's Bill, if passed, would give those with a terminal illness or a grievous or incurable medical condition the option to request a medically assisted death.
Mr English said the criteria for who would be eligible to request assisted dying was too broad, the test for whether a person had consented to die was too low, and there were too few consequences in place for anyone who broke the safeguards of the legislation.
Dr English, who is a GP and runs a medical practice in Kelburn, said legalising euthanasia would put pressure on patients to not be a burden to their loved ones.
She said the Bill would give doctors unprecedented power over life and death.
Both of the Englishes linked their arguments to the campaign to address New Zealand's youth suicide rates.
"How can we say it's a bad thing for a young person to take their life but a progressive thing for a sick old person to think about taking their lives when 17 and 18 year olds feel the same existential pain, probably more, than the adult?" Mr English asked.
Mr Seymour said he found the submission from the Englishes "disappointing".
"It's the same old chestnut that 'we can't prove anything but we think that there might be something going on'. That's what we see all around the world, is the opponents of choice run this fear, uncertainty and doubt campaign," he said.
"There are people who've done scientific studies over 20 years of jurisdictions and they do not find evidence of the coercion that he speculates."
Mr Seymour said there's "no evidence" that legalising euthanasia leads to a rise in suicides.
The End of Life Choice Bill passed its first reading in December with 76 MPs voting in favour and 44 against.
The select committee stage, due to close in March, was extended until March 2019 after an unprecedented 35,000 submissions were received.
The committee is now travelling around the country to hear people's views on the bill.
The vast majority (71 percent) of New Zealanders support euthanasia, a Newshub Reid Research poll found in February.