In a quiet Adelaide suburb on a sunny autumn day, Gwen Nitschke is sitting at her kitchen table. She is talking about her son, Philip, who for the past ten years has been campaigning in the cause of voluntary euthanasia, and in the meantime doing as much as he can to assist the many people who ask him for help. Very early in the conversation his mother leans forward and looks me sternly in the eye. “Can I askyou a question?” she says. “Are you pro or anti?”

I have with me a copy of Killing Me Softly, the new book her son has co-written with sociologist Dr Fiona Stewart. It gives some background on his own life and career – including an affectionate paragraph about his mother – and traces the history of the voluntary euthanasia movement in Australia. It spells out an elegant, detailed and damning argument about the way that death in our society has become the jointly managed property of the medical profession, the legal profession and the church. It outlines Dr Nitschke’s belief in the desirability of choosing to end one’s own life independently, in freedom and dignity. It explores the possibility of a DIY “peaceful pill” that would bypass issues of legality. And it foreshadows the pressure he now feels to move overseas, with the government preparing to take control of the Senate on July 1 and to outlaw even the current restricted activities of Dr Nitschke’s Exit International organisation.

“I’m pro,” I say. “If I were anti I would never have had the nerve to invite myself into your house.”

Gwen Nitschke’s 84 years, many of them spent under the blistering rural South Australian sun, have had their way with her, but her gaze is still clear and shrewd. It’s almost unthinkable that anyone opposed to her son and his campaign would invade the privacy of a fragile elderly woman and pretend to be sympathetic in order to gather material to use against him. But it’s been done before, she says. She doesn’t want to be caught out again.

She too has read Killing Me Softly. Does she agree with her son? “Absolutely. Absolutely and wholeheartedly.” She has lost patience, she says, with some of the more timid elderly people she knows, who seem to fear that the legalisation of voluntary euthanasia would lead to some kind of ritual mass slaughter of everyone over 60. “I used to speak to people round here about it, but I have given up. They say: ‘It might get out of hand – the young people might start doing away with the old people.’ It’s as if they’ve just got no understanding of the meaning of the word ‘voluntary’.”

I wonder whether the old have always been this frightened of the young. Not that Gwen herself seems frightened of anything at all; though physically frail and able to move around only with difficulty, she gives the impression that if she were to find some off-his-face teenage malefactor breaking in through the bathroom window in the middle of the night, she would hit him over the head with her walking frame and then give him a good talking-to once he regained consciousness.

Does Philip take after her? “He’s like his dad in lots of ways, but he’s softer than his dad. He’s got a mixture of Irish and German blood – Irish for fighting causes and German for tenacity.” I ask what he was like as a child in the 1950s when both his parents were teaching in rural South Australian schools. “I took him to school at the age of three because I started teaching then. He was my youngest. And he sat up the back and read his primer and did his arithmetic, and he went up to grade two with the rest of them. When we moved to Peterborough and he started school up there, the teacher rang up and said: ‘You’ve got Philip’s age wrong – he’s in grade three but it says here that he’s only six.’ I said: ‘Yes, that’s right.’”

Somehow it seems an unlikely trajectory for a man who by his mid-twenties had a PhD in laser physics, but it’s clear from the autobiographical sketch at the beginning of Killing Me Softly that Nitschke is by nature an advocate and activist, a social reformer, a believer in the relief of suffering. When the Northern Territory’s chief minister Marshall Perron proposed the legalisation of voluntary euthanasia in the mid-1990s Nitschke, then a GP in Darwin, approved of the idea and was roused by the Australian Medical Association’s resistance to it. It was then that his own advocacy and activism in the cause began. During its nine fleeting months of legality in the NT, four terminally people – with Nitschke’s help and support – took their own lives, using the computer-based “Deliverance Machine” that gave them full control over what would happen to them.

A few days before I go to visit his mother I hear him on radio, responding to a hectoring, belligerent attack from a talk-show host. Nitschke by contrast sounds calm and firm, with just a hint of well-controlled irritation, like a single turn of the pepper-grinder. In the opening chapter of his book he says its purpose is to help him stop saying sorry to the many desperate people who ask for his help. “After eight years of apologising, I can apologise no more.” The nature of Nitschke’s work means that even when it’s successful – perhaps especially when it’s successful – it’s a source of ongoing strain.

“On the few occasions when he was able to use that machine in the Northern Territory he got terribly upset,” says Gwen, “because he’d got to know those people. And he gets very tired at times. I think it’s taking its toll.”

I ask whether she ever wishes he’d go back to leading an ordinary, peaceful life. “No, never. I’m very proud of what he’s doing.”

Does it upset her when he is attacked by people who oppose his work? “It annoys me. It makes me angry. But Philip is well and truly good at looking after himself and he always has been. So it doesn’t really upset me because I know he can come back, he can handle it. I get angry, but after I cool down a bit I think of the thing we used to say when we were kids – sticks and stones may break my bones but names can never hurt me.”

I’m not so sure she’s right about this, but I decide not to argue the point. I ask instead whether her own experiences of dealing with death have influenced her attitude. “Well, my sister died about two years ago. She was absolutely terrified that she would end her life in a hospital bed; every time we talked on the phone she used to go on and on about it. Then she had a seizure one morning and they put her in an ambulance but she died on the way to hospital. I was upset, of course, but I didn’t feel too sad about it, because I knew she wanted to go that way.”

She has lost two sisters and a brother, but behind us in the living room there’s a whole wall of family photos: her three children, four grandchildren and four great-grandchildren. She is writing her memoirs, and she shows me a thick hard-backed exercise book that’s three-quarters full of her own clear handwriting. On the most recent pages the writing, while still neat and legible, is more shaky and tentative; when the arthritis in her right hand defeated her, she taught herself to write with her left and kept on going.

So entertaining is her conversation, so spirited her support of her son and so beguiling her Irish gift for storytelling that I lose track of the time. As I apologise for tiring her and hustle myself out the door, I mention that I was impressed by Philip on the radio. “Yes, he’s good with words,” she says proudly. “The book was very well written, I thought. And he talks well.”