Sean Davison received the single-line email at 1pm on Wednesday, May 28, 2008, 19 months after the funeral. He hadn't seen his sister Mary since they had posed for a photograph together in front of the coffin, which they helped paint in the bright colours, black cats and beaches so loved by their mum.

"I've had enough, this is not life," Patricia, 85, had repeatedly told her four adult children from her sick bed in late 2006. Cancer had spread to her lungs, liver and brain. Patricia – a former GP and psychiatrist who loved painting and dancing – was in constant pain and desperate to die. She jokingly asked to be drowned in Otago Harbour, which she could see from her home near Dunedin, on New Zealand's South Island.

She stopped eating to hasten her demise but was still in a pitiful state 33 days later, on October 25. On her request, Davison, who had travelled from his home in Cape Town to nurse her, crushed up the dozen morphine pills she had stockpiled. "You are a wonderful son," she told her youngest child, after he helped her drink the lethal dose in a glass of water that she lacked the strength to grasp.

Davison says his three older siblings – Fergus, Mary and Jo – were relieved their mother was no longer suffering. But his subsequent decision to publish a memoir about her death opened a wound in his family that is unlikely to heal. He will discuss the rift at next week's Exit International conference in Melbourne, amid a growing push for pro-euthanasia laws in Victoria.

"Assisted death and dying divides families, because some members of a family find it very difficult to let go of a loved one," Davison says.

At the time of his mother's death, he enjoyed a close relationship with Mary, a gerontologist who specialises in dementia. But she twice took legal action to stop the publication of his memoir, arguing it was libellous and breached her family's privacy.

In May 2008, while Mary was working at a Melbourne hospital, she emailed Davison that she would tell the police "you murdered our mother". He never imagined she would act on her threat. But while visiting Dunedin, in September 2010, he was arrested and charged under laws prohibiting assisted suicide.

Davison's memoir Before We Say Goodbye, published in 2009, omitted his role in his mother's death at the request of his publisher. But an earlier draft that he gave his siblings to read detailed her overdose. He recalls the moment a police detective placed the damning manuscript before him. On the cover was the photograph of his mother that he had specially glued to Mary's copy.

In November 2011, the New Zealand High Court sentenced Davison – who the judge described as an "exceptionally devoted and loving son" – to five months' home detention for "counselling and procuring" his mother's death.

The first time I spoke to him  was by phone in January 2012, while he was serving his sentence at a friend's home in Dunedin, more than 10,000 kilometres from his wife and young children in Cape Town. His soft voice shuddered then as he spoke of his agonised decision to help his mother die.

Four years on, Davison, 54 and professor of biotechnology at the University of the Western Cape, sounds stronger and looks well, with his lean frame and bright blue eyes. He speaks on Skype from his home near Table Mountain, which he climbs on weekends. One of his mother's landscapes hangs on the wall of his study, where he escapes the ruckus of his children Flynn, 7, Finnian, 6, and Fia, 2.

"It was very hard to be separated from my family when the children were so young. It was one of the happiest days of my life to get back to them," he says of his time in detention.

He accuses Mary of the "horrendous crime" of sending his manuscript to the police. Mary, who now works in Wellington, did not respond to requests for comment. But she denied the allegation when I spoke to her briefly by phone in 2012. "He is entitled to believe that. It didn't come through me," she said. "I love my brother."

I also spoke then to Fergus and Jo, who both praised their brother's "bravery" in assisting their mother's suicide.

Davison, who has not spoken to Mary for several years, says he was initially angry at his sister. But he misses her. Do you forgive her, I ask. "Normally, forgiveness requires some kind of regret or apology from the person wanting forgiveness. But in spite of that I do forgive her."

His life has taken a sharp turn since his incarceration. Davison is now president of the World Federation of Right to Die Societies and founder of Dignity South Africa, which wants to help overturn the country's prohibition of assisted suicide. The campaign was buoyed in May 2015, when the High Court in Pretoria granted a 65-year-old terminally ill man the right to end his life with the help of a doctor. The South African government has appealed the decision.

A similar push in Victoria to legalise euthanasia won public support this week from several government ministers. Premier Daniel Andrews stopped short of supporting euthanasia but said the recent death from cancer of his father had challenged his views against assisted suicide.

Assisted suicide is illegal in NSW. In 2013, the NSW Parliament rejected a bill to allow terminally ill people with decision-making capacity to request medical assistance to die.  However a 2015 survey found 72 per cent of people in NSW agreed that terminally ill patients should be able to legally end their own lives with medical assistance.

Davison says seeing the suffering of a family member often changes people's position. "It is the core of our humanity not to let people suffer. Any humane person in the same situation I was in with my mother would have done exactly the same thing," he says. "I am certain that in 10 years' time the Western world is going to look back at cases like mine and say 'How horrific, somebody could be criminalised for an act of compassion'."

He admits helping "quite a few" people die, including a quadriplegic man in constant pain who overdosed on a sedative in November 2013. He expects his own wife and children would do the same for him in similar straits. "If I felt life wasn't worth living – and quite likely I wouldn't if I am a quadriplegic – I would ask for help. And I believe my children, if they were a bit older, would help me, because they are kind, compassionate children."

And if the roles were reversed? If one of his own sons broke his neck, was paralysed and no longer wanted to live? "If my son begged me to die, extremely painful as it would be, I probably would on his request."

It's difficult enough to let a loved one go, let alone to play an active role in their death. Davison advises families to openly discuss such issues and to respect each person's wishes about their death. "To me, it's a logical issue and the issue is the individual and their right to choose," he says.

"My mother's death was actually nothing to do with me in the end. Sure I had to carry out the act but it was what she wanted and to deny her would be me playing God. If I had not answered my mother's pleas to help her to die, if I had left her suffering because I didn't have the strength to help her, I would have regretted that for the rest of my life."

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