One thousand doctors have signed a letter saying they "want no part in assisted suicide".

They have urged politicians and policy-makers to let them focus on saving lives and care for the dying, rather than taking lives, which they deemed unethical - whether legal or not.

The letter comes as Parliament is due to start the second reading debate on Act MP David Seymour's End of Life Choice Bill on Wednesday.

The doctors said they were "committed to the concept of death with dignity and comfort", including effective pain relief and excellence in palliative care. And they uphold the right of patients to decline treatment.

But the 1000-strong group said it believed "physician assisted suicide and euthanasia are unethical, even if they were made legal".

"We believe that crossing the line to intentionally assist a person to die would fundamentally weaken the doctor-patient relationship which is based on trust and respect," the letter reads.

"We are especially concerned with protecting vulnerable people who can feel they have become a burden to others, and we are committed to supporting those who find their own life situations a heavy burden."

Finishing, they said: "Doctors are not necessary in the regulation or practice of assisted suicide. They are included only to provide a cloak of medical legitimacy.

"Leave doctors to focus on saving lives and providing real care to the dying."

The letter - along with 1000 names of the doctors who support it - has been published as a full-page advertisement in today's Herald on Sunday.

The advert was taken out by the Care Alliance, a charitable trust committed to nurturing better conversations about dying in New Zealand, according to the website: doctorssayno.nz

It lists the names of all the doctors who have supported the letter, and urges others to sign up too.

The letter publication comes after a Canadian doctor who has taken part in champagne parties before administering lethal drugs to her patients urged Kiwi MPs to support the vote to legalise euthanasia.

Dr Stefanie Green, who has personally euthanised 150 people since Canada legalised assisted dying in 2016, has been brought here by the End of Life Choice Society.

The bill passed its first reading by a massive 76-44 margin in December 2017, but MPs opposing the bill predict a dramatically tighter margin of less than five votes on the second reading.

Seymour says the second reading debate is likely to start at 7.30pm on Wednesday, with a vote expected around 10pm.

Green, a former maternity doctor, said she stepped up to help people wanting to end their lives because other doctors were unwilling to do it after Canada legalised euthanasia. Euthanising people is now 90 per cent of her practice.

"We have had about 8000 assisted deaths, with no charges against any physicians for any abuse of the system," she said.

Patients had to meet "rigorous criteria", she said, and about a third of people who requested euthanasia did not receive it.

But she said the actual deaths for those who were approved were usually in "a very beautiful environment".

"For some people it's quiet, and for some people we've had 50 people and champagne - quite often there is champagne," she said.

"There is often a celebration of life just before the death. Assisted death enables people to have a chance to say goodbye to their loved ones and for their loved ones to say goodbye to them."

As currently drafted, Seymour's bill would allow New Zealanders to request assisted dying if they have a terminal illness or suffer from "a grievous and irremediable medical condition".

However, if the bill passes its second reading, Seymour has promised to amend it in its clause-by-clause committee stages to limit euthanasia only to people who have "a terminal illness that is likely to end the person's life within six months", and to state that age, disability or mental illness cannot be reasons to grant consent.

Green MP Julie Anne Genter said the Greens' policy committee had instructed all Green MPs to support the second reading, but the Greens would not support the third and final reading unless Seymour's proposed amendments were passed.

New Zealand First MP Tracey Martin said her party would also support the second reading but would only support the third reading if Parliament passed an amendment she proposes to put the bill to a binding referendum at the next election.

A Horizon poll for the End of Life Choice Society last month found that 74 per cent of New Zealanders would support a right to euthanasia for terminally ill people.