One of the committee's members, Labour MP Louisa Wall, said his argument was "fundamentally flawed" because he did not differentiate between medically-assisted dying and suicide.
"I don't see them as congruent," she said. "There is a huge contrast between people who are facing imminent death and people who are hopeless or depressed."
"To say that someone like [euthanasia advocate] Lecretia Seales was committing suicide is just wrong."
Seales, a Wellington lawyer, took an unsuccessful case to the High Court to have a legal, medically-assisted death earlier this year.
McCoskrie said any discussion of suicide should focus on prevention.
"In complete contrast, this inquiry is initiated and is driven by a desire to promote assisted suicide. You don't discourage suicide by assisting suicide."
The committee also heard from Caritas, a Catholic agency which opposed voluntary euthanasia.
Advocacy and Research Manager Lisa Beech said some New Zealanders were exploited in their workplace, as tenants, and in their homes - despite legal protections.
In all of these situations, people who apparently had a choice were actually under huge pressure, she said.
"We must be absolutely certain that the powerlessness, exploitation and lack of choice that many New Zealanders face in homes, workplaces and communities is not present in the hour of their death."
"We oppose assisted suicide because something that may appear a choice for one person may be a decision made under pressure for another."
The committee hearing from 1800 submitters before it makes recommendations for the Government.