"How can our country allow this to happen in what we consider to be a civilised society?"
Davison gave his mother crushed morphine tablets in water on October 24, 2006.
He then wrote about her death in the 2009 book, Before We Say Goodbye.
Outside the court yesterday, he described the court process and his plea on the alternative charge as "a very stressful experience".
He said he had never denied assisting the suicide of his mother.
Other euthanasia campaigners would say yesterday's court decision was "a victory for the cause", Davison said.
"Maybe in the days to come I will agree with them. But for me, my family and the memory of my mother, today's court decision is a personal tragedy."
Justice Christine French remanded Davison for sentencing on November 24, directing that the pre-sentencing report consider the possibility of an electronically monitored penalty.
Davison's book did not give a precise account of how he helped his mother to die. But the original manuscript, which was leaked, told how he gave her a lethal dose of morphine.
He admitted as such in an interview with the Herald on Sunday in 2009.
His case reignited the euthanasia debate in New Zealand.
Voluntary Euthanasia NZ secretary Pat Hubbard said Davison's plea had taken the group by surprise.
"He clearly didn't want to murder her in the ordinary sense of that term. He came to help her. And what she wanted help with was dying."
Right to Life spokesman Ken Orr said he had compassion for people in Davison's position, but voluntary euthanasia was a "dangerous road".
- Otago Daily Times, additional reporting: Nicholas Jones
