Surveyed doctors say they played a role in hastening demise of dying patients through drugs or treatment.
Sixteen GPs have admitted their involvement in the deaths of patients who were prescribed medicines explicitly to shorten their lives.
All the patients were near the end of their lives: 15 faced death within a week and one was expected to survive one to four weeks.
The GPs "attributed death to a drug that had been prescribed, supplied, or administered explicitly for the purpose of hastening the patient's death", according to the results of a survey of GPs published in today's New Zealand Medical Journal.
The survey results follow the decision of Parliament's health select committee this week to hold an inquiry into voluntary euthanasia.
A petition from the Voluntary Euthanasia Society was presented to Parliament by supporters including Matt Vickers, the widower of Lecretia Seales, who died last month, aged 42, from a brain tumour. She was unsuccessful in seeking a High Court ruling that would let her doctor help her die without criminal prosecution.
Euthanasia and physician-assisted suicide are illegal. But the survey of GPs provides few details of the life-shortening drugs which were given - in some cases by more than one person - by two doctors, 15 nurses and one "other" person.
The exception is the GP who reported involving a hospice home-care team, say the Auckland University researchers, Dr Phillipa Malpas - a member of the Voluntary Euthanasia Society - and her colleagues.
"The actions this respondent described - increasing sedation and reducing fluid intake to hasten the end of life - have been questioned in the literature as 'slow euthanasia'," they wrote in the journal.
The survey was completed by 650 of the 3420 GPs it was sent to, of whom 359 had made an end-of-life medical decision with the most recent death of a patient in their care in the preceding 12 months.
The last action before death ranged from decisions to withdraw or withhold treatment or intensify the alleviation of pain and/or symptoms with the likelihood life would be shortened - through to actions partly or explicitly intended not to prolong life or to hasten death.
Auckland University professor of psychology Glynn Owens told the Science Media Centre the survey showed that criminalising assisted dying did not prevent it. "The fact that practitioners are willing to risk their careers and their liberty by acting illegally reflects the severity of the perceived need in those cases."
But Medical Association chairman Dr Stephen Child said he suspected most of the 16 GPs' patients would have got increased doses of morphine for pain relief, with the consequence that life was shortened. This was ethical and was not euthanasia.
GPs had attended a patient who died in the preceding 12 months
359 of them made an end-of-life medical decision
316 intensified treatment to reduce pain/symptoms, knowing the treatment was also likely to hasten death
48 withdrew a treatment explicitly to not prolong or to shorten life
16 said the death was caused by drug prescribed, supplied or given explicitly to hasten death