Davison also revealed yesterday that there are a number of doctors in the country who secretly help terminally ill patients end their lives.

He yesterday addressed the Cape Town Press Club, where he spoke candidly about his arrest and his mother's death.

"New Zealand is a nanny state, so along comes this person who is a mercy killer. Now they have got a real issue to deal with, and boy, they dealt with it," he said.

"My case was perceived as a man who published a book about an act of compassion in helping his terminally ill mother die at her request. It was all very clear and compassionate. What did they do in New Zealand?

"They put eight detectives on the case, presuming all eight had to buy a copy of the book and read it, which is very good for the book sales, and they interviewed every single character in the book.

"The police made one very big mistake at the very beginning: they didn't ask me. Yet I never denied helping my mother to die. Instead they launched this investigation and arrested me for attempted murder."

He described the police investigation as heavy-handed, saying it cost the government an estimated R4-million while he spent more than R300000 in legal fees.

Davison is the head of the forensic laboratory at the University of the Western Cape.

His 85-year-old mother, a retired doctor, had asked Davison to help her die. She was diagnosed with cancer in 2004 and he gave her a lethal dose of morphine in 2006. He went on to write a book about it, Before We Say Goodbye. Retired archbishop Desmond Tutu wrote to the New Zealand court to ask for a lenient sentence.

"She asked me to come to New Zealand to spend the final weeks of her life with her. When I went there, her health had steadily deteriorated. She slowly lost her ability to read and to paint - two of her passions. Her ability to walk was also becoming less and less easy," said Davison.

"She knew the ghastly death that awaited her from secondary cancer, so she made a decision to go on a hunger strike because she thought if she stopped eating there would be one possible outcome - she would die very, very quickly and she wrote a living will outlining this plan.

"We, her children, supported her because she was a medical doctor and she was our mother.

"Tomorrow came and left and, after five weeks, she was decomposing on her own bed, unable to move any limb, unable to end her own life.

"And that is the only reason she asked me."

In December 2010, Davison established Dignity South Africa, which lobbies for a change in the law to allow assisted dying.

The organisation this week invited people to sign a petition calling for a bill to be brought before parliament that would legalise "assisted dying in precisely defined conditions".

"If we don't have a law change you're playing Lotto with your doctor. You might think you've got a doctor who is going to help you and you don't find out until the very end and it's too late to change doctors.

"If the law was changed, these doctors who are doing it in South Africa, they wouldn't have to do it behind the scenes illegally, they would be doing it legally," Davison said.