It was late evening on a Monday in April, the end of a bright autumn day in Johannesburg, when Mum died. The nurses who were caring for her had made her comfortable for the night, turning her on to her side with a pillow to cuddle, the way she liked to sleep. My sisters Vicky and Jassy and I were with her. We held her hands, told her how much we loved her, and that it was safe and right for her to go. After a few minutes, her breathing stopped. It was a good death – the kind of death I think most people would choose if we could: free from pain and surrounded by love. She wasn't hooked up to tubes or monitors; she was even wearing her own pretty nightdress rather than a hospital gown. I suppose it's what you'd call a natural death. But in one way Mum's death was exceptional, shocking even. She had decided to die, about 10 days before, and for the previous six days had had no food and almost no water. She had chosen to die of thirst.

The story of Mum's death begins in September 2008, when she sent an email to her daughters, telling us that she had been diagnosed with colon cancer. "Don't worry about me, darlings," she wrote – but it was clear that she herself was desperately worried: about loss of dignity, about the cost of her medical care, about whether Dad would be able to learn to make his favourite spaghetti bolognese if she was unable to cook for them both – all the details of death. In the event, after a frightening and rocky few weeks and major surgery, Mum recovered well, and by the beginning of this year we had all started to make plans to celebrate her 80th birthday in August. With Mum and Dad's five daughters living so far apart, times when we were all together are rare and precious.

Then, at the end of February, came a letter from my eldest sister Emma in place of Mum's normal Sunday email. "Mum isn't feeling great," she wrote, "and she asked me to write so you wouldn't worry." But, of course, we instantly did begin to worry, and it was only a few days before the diagnosis we'd dreaded arrived. Mum's cancer had metastasised and spread to her brain and possibly also to her lungs. Her oncologist immediately set her on a course of radiotherapy and cortisone to shrink the tumours, telling us that it would take up to two weeks before we knew whether the treatment had been effective.

Vicky, who has experience caring for the terminally ill, travelled to Johannesburg to help Mum and Dad, and they had long and honest talks about the decisions Mum faced. She wasn't quite ready, she said at first, to leave Dad, "this beautiful world", as she put it, and us. But over the next few days, Mum's condition began to deteriorate, and she became increasingly uncomfortable and frustrated. Mum was a very private person. She loved order; she hated mess and squalor, and illness is often messy and squalid. She told Vicky again and again that she didn't want to go through more treatment. She didn't want Dad to see cancer killing her. She wanted to die quickly and quietly. She didn't want any fuss.

It was Emma who first told Mum about an article she'd read several years earlier in the Journal of the American Medical Association about voluntary refusal of food and drink as a means of bringing about death in the terminally ill. The author described it as a relatively painless way to die: a gradual slipping away into a coma, followed by death. Typically, a person will take between five and 10 days to die this way and, crucially, they have the option to change their mind and stop the process until almost the end. Mum just latched on to the idea, Emma remembers. She decided that she was going to stop taking the cortisone that was keeping the tumours in check, and stop eating and drinking as well. She wasn't going to let the disease take over.

Like many people, I simply didn't know that choosing to stop eating and drinking is, as Jassy put it, "a Thing". But it is – it's a thing about which you can find pages and pages of information on the internet, once you have a reason to Google it. It's a thing that's been reported quite extensively in medical literature, known variously as VSED (Voluntary Stopping Eating and Drinking), VRFF (Voluntary Refusal of Food and Fluid), PRNH (Patient Refusal of Nutrition and Hydration), or self-dehydration. It's a thing doctors who have practised for a long time and seen many patients die at home describe as "taking to their bed" or "turning their face to the wall". It's a thing that was explored and argued by lawyers acting for the man known publicly as Martin, whose case was recently heard alongside Tony Nicklinson's. It's a thing, as I discovered when researching this article, that people seem afraid to talk about.

‘She wanted the kind of death most people would choose, if we could’ (from left): Jassy, Juliet, Sophie, Emma and Vicky with their parents. Photograph: Courtesy Sophie Mackenzie

There is nothing illegal about refusing food and drink; in fact, feeding a competent person against their will constitutes assault. Nonetheless, Mum was afraid that something would happen to prevent her reaching her goal. She worried about falling and breaking a hip, ending up in hospital and being caught in a spiral of intervention that would prolong her life. She worried that intravenous nutrition and hydration would be given to "keep her comfortable" if she became unconscious. She had signed a living will expressing her preferences, but such advance directives are generally regarded as worthless unless the dying person has a supportive medical team who understand their wishes and are prepared to act upon them.

South Africa's legal position on assisted dying is fundamentally the same as Britain's: if you help another person bring about their death, you could risk prosecution. As in the UK, there have been attempts to clarify the law on end-of-life decision-making processes, removing the grey areas that currently exist. Professor Willem Landman, CEO of the Ethics Institute of South Africa, is one of the leading figures campaigning for a change in the law. He explains that in 1998, President Nelson Mandela commissioned a report into the legal and ethical issues surrounding assisted dying, assisted suicide and euthanasia, but after being tabled in parliament in 2000, the report was ignored by the ministry of health. Landman believes this may be because the then minister of health, Dr Manto Tshabalala-Msimang, regarded the issue as something of a middle-class problem. Perhaps it is. When birth and death are less medicalised and a more integral and familiar part of life, perhaps choice in such matters seems an unnecessary luxury. Perhaps mothers do die gently, cared for by their daughters, without worrying about indignity and fuss. But they don't die quickly, or without pain.

Mum had always talked frankly to us about death. My sister Juliet remembers when she was a little girl and terrified of dying, Mum telling her, "Your body is like a suit of old clothes – by that time you will be tired of it and you won't want it any more." Mum hated clutter and hoarding; when a thing had outlived its usefulness, it was to be disposed of without sentiment, whether it was a favourite dress the moths had got to, a book that was falling apart at the spine or her own body. She made it clear to us that she would far rather be dead than be a burden on her husband and daughters. Now, with her hands becoming increasingly unsteady and fits of dizziness coming unexpectedly, she was finding the colostomy bag she'd had since her surgery in 2008 difficult and humiliating to deal with. She felt that her body was wearing out and, more frighteningly, her head felt heavy and fuzzy.

Over the course of a long phone call, Vicky told me that Mum had made a decision. She was going to stop taking the medication that was checking the spread of her cancer, and stop eating and drinking, too. We had about a week to say goodbye, so Juliet and I flew to Johannesburg as soon as we could. It was the sort of family reunion Mum loved, and in many ways it was like all the other happy times we have spent together, filled with love and laughter. We planned meals together, cooked in Mum and Dad's tiny kitchen, ate and drank together, just as we've always done on family holidays, over Christmases and at other celebrations – only this time it was Mum's life we were celebrating, while she went through the process of dying.

It was the most remarkable week of my life, and it has left me unafraid of death.

I'd been afraid of seeing Mum since this latest illness. I was expecting to find her diminished, somehow less herself. But I was amazed by how well she seemed. Vicky had cropped her hair so that she wouldn't have to experience it falling out, and Mum, with the pride in her looks that beautiful women never lose, was obviously upset by her appearance. But to me she looked lovely. She shone with happiness at having all her daughters there with her and Dad. She was full of her usual curiosity and humour. She didn't look like someone who was ready to die, and this frightened me – what if somehow stopping eating and drinking didn't work, and Mum lingered on in pain? What if the process didn't take a matter of days but a matter of weeks? Part of me knew how much she would hate to be thwarted in her intent; part of me longed to keep her with us for longer, and hoped that, even now, some sort of miracle might happen. I even suggested to Mum that she should stay at home for a few days, just to see how she felt, but she was adamant: her decision had been made. The next day, she was going to move to the nursing home at the retirement community where she and Dad lived, and she was going to die there.

Although Mum was unable to walk unsupported and her hands were very shaky, she was alert and lucid, chatting to us as she always had. Her appetite was small, but she was still enjoying food: pizza with olives and anchovies, strong cheddar cheese, the fruit cake Juliet had baked and been saving for Mum's 80th birthday. Juliet's cake was the last thing Mum ate, on Wednesday 28 March, when her brother visited to say goodbye, and she had a last chat with her sister in the UK. We were all in tears that morning, except for Mum, who stayed calm and cheerful.

I didn't see Mum leave her home for the last time. Dad, Emma, Juliet and Vicky were with her, and Juliet cried afterwards when she told Jassy and me how Mum had gone around the little house where she and Dad had been so happy, saying goodbye to it and all her precious things, before being driven the few hundred yards to the nursing home. The staff there welcomed her and settled her into bed, and she curled up and went to sleep.

It's fortunate that Mum was surrounded by people who supported her decision. Her GP believes that people at the end of their lives should have the right to be "conductors of their own orchestra", and made it plain that she would prescribe whatever pain relief Mum needed to mask the symptoms of her advancing cancer, and whatever sedation she required to alleviate distress. The management of the nursing home hold the same view. Her GP gave them written instructions explaining Mum's decision and outlining the care she would need. "She is a very brave lady," the email concluded. And, of course, Mum had five daughters who adored her and a husband who'd worshipped her for almost 60 years, and we were all unequivocally on her side.

The five of us sisters rallied behind Mum and Dad; we were a team. As I write this, it's difficult to isolate my own feelings and experiences from those of my sisters – who said what, who made decisions, who cried. It's a gruesome analogy, but the five of us, Dad, Mum's GP and her carers were like a cheerleading squad urging her on towards the goal she had chosen. This isn't always the case. I was horrified to read a 2011 report in the New York Times which told the story of Armond and Dorothy Rudolph. The Rudolphs, both in their 90s and with failing health, made the decision to end their lives together by stopping eating and drinking while they were still mentally competent and able to live independently. The management of the assisted-living facility where they had their home responded by attempting to evict them. It was only after intervention by the couple's family, the local press and medical and legal experts that the attempt was stopped. Armond Rudolph died after 10 days without food and drink, and Dorothy a day later.

We laughed a lot over the next few days. We teased Juliet about developing late-onset colour-blindness when she kept putting bottles of red wine in the fridge, while the white stayed warm on the kitchen counter. Jassy and I speculated macabrely about what would happen if the nurses forgot to take down the "Nil by Mouth" sign over Mum's bed after she died – we imagined some poor old dear being moved into her cubicle and waving plaintively as the tea trolley rattled unheedingly past. Emma and Juliet still haven't let me forget the nightmare 10-mile walk I dragged them on around the suburbs of Johannesburg, possibly the world's least pedestrian-friendly city. One night after dinner, Emma and Juliet went to visit Mum, and the three of them chatted and reminisced until eventually my sisters were asked to leave – their laughter was keeping the other old ladies awake.

In the midst of Mum's death, our normal lives went on. I was writing a seemingly endless list of product descriptions of cheeses – I'd hoped that Mum would have enjoyed working with me on them; a writer herself, she would have embraced the challenge of describing the differences between stilton and roquefort. But she was too drowsy, although she loved being read to. We planned each day as it came: who would be spending time with Mum, and when; who would cook, and what, and for how many. Of course we talked about Mum and what was happening, sharing our fears about what the next few days would hold, and our happy memories of her. Often, it felt just as if she was there with us, as she always had been when the family was together.

There were hard parts, too. By Friday, although Mum was still spending a lot of time asleep, her waking hours were difficult. She was dreaming, she told us, about drinking ice-cold Coke, and waking feeling desperately thirsty, as one does with a bad hangover. Chips of ice helped to relieve the dryness in her mouth, and she had sugar-free mints to suck as well as frequent glycerine mouth swabs, but the thirst remained. She also developed a pounding headache as her blood sugar level fell. Emma bought her some Coke, thinking that the occasional sip would make her more comfortable and do little or nothing to prolong her life. But Vicky, determined to support Mum completely in her decision, said, "It could set her back days!" and took it away, leaving the nurses crying in her wake. That day, Mum said to us, "I didn't realise it would be so hard."

She wasn't the only one. Being a spectator at the death of someone you love is bitterly hard. We expect medical science to intervene to relieve suffering, and to a great extent it does. But the journey is a lonely one. Even surrounded by the people who loved her most, and professionals who gave her the very best palliative care, Mum fought her battle for death alone. How much more lonely, and how much more frightening, the process must be for people who aren't supported by their family, or aren't able to communicate their wishes, I can only imagine.

Mum spent a lot of the next three days asleep, but when she was awake, we chatted to her and to each other, and Jassy read to her from her new novel, which is dedicated to Mum. She also enjoyed having her hands massaged with her favourite body lotion as her skin began to get dryer, and the scent greeted us when we came into the ward to see her. Dad was his usual self: strong, serene and often silent, surrounded by his five chattering daughters, but occasionally cracking us all up with his characteristic dry wit. He spent a lot of time at Mum's bedside, sitting with her while she slept. Mostly, it was a peaceful and happy time.

On Sunday, Mum began to slip away. In the morning she was still alert, asking for ice to suck and chatting to us almost normally, but by the evening she was no longer able to speak easily. I'd had a bad cold for the previous few days, and when I heard the new rattle in Mum's breathing, I became convinced that she had caught it and my virus was going to be the thing that finally carried her off. I sat by her bed and cried my heart out – a mixture of guilt, grief and all the stress of the last few days just pouring out of me. I felt embarrassed by my tears in the face of Mum's courage – absurdly, I didn't want to worry her. She asked me several times, "Am I going to be all right?" I told her that she was, and that I loved her. There wasn't anything else to say. That was the last time she spoke.

For most of the six days it took her to die, Mum was in no pain. The morphine patches she had been prescribed relieved any symptoms her advancing cancer might have caused, and the sedatives kept her calm and drowsy. However, it's not known whether such palliative care actually removes the dying person's experience of hunger and thirst, even once they have lost consciousness. A paper published in the Journal of Medical Ethics in 2011 concluded that "continuous deep sedation may blunt the wakefulness component of human consciousness without eradicating internal affective awareness of thirst and hunger". I hope that, for Mum, the dreams of ice-cold Coke and the sensation of thirst stopped once she fell asleep on that Sunday night.

On Monday morning, Mum was moved from the main ward where she had spent the past five nights. The curtains around her bed had remained closed while she was there – Mum would have wanted the privacy, we knew, but still we speculated darkly about whether the staff had kept her hidden in case the other ladies got ideas and decided to emulate her. Her new bed was in a private room. She barely woke when she was lifted, and she was losing the ability to swallow. In the afternoon, the vicar came to say the last rites, with all five of us sisters and Dad there, perching on her bed and chairs around it. Mum seemed to be aware of the words of the service – perhaps owing to the vicar's maximum-decibel delivery, which we all laughed about afterwards – but she was sliding deeper and deeper into unconsciousness.

That evening passed slowly, as time does when you're waiting for something important to happen. I carried on with my writing – the cheese product descriptions were complete and I'd moved on to fish, and it was comforting to escape into work. Emma and Juliet were spending the night at home with Emma's partner and his children. Dad did a crossword puzzle; Vicky made a batch of chicken soup, using Mum's recipe. Over dinner we talked about the practical things that would soon need to be done, and decided that Vicky would be the one to phone the undertaker, and that she would remove and take care of Mum's rings and watch. We asked Dad if he felt it was important for him to be there when Mum died, but he felt that he had already said a very temporary goodbye – he was, and remains, confident that they will be together always once he joins her. So Jassy, Vicky and I took the familiar route up to the nursing home to see how Mum was and decide what to do about spending the night with her. It was the last time we'd walk that way; within a few minutes of our arrival, she was dead.

I went back to Jassy's house that night and wrote about Mum's death. My overwhelming feelings were relief and pride – my brave, beautiful mother had taken on the hardest challenge of her life, and won. I could go home to my partner, whom I missed desperately, my home in London and my little cat. My sisters and Dad felt the same, I think: tired, but triumphant, too. We'd achieved what we had set out to do, as sisters and as a family.

Even if Dignitas had been a practical option for Mum, I don't believe she would have chosen it. Apart from anything else, she loathed travelling. (I suspect she would also have regarded the cost as a wanton extravagance.) But what if there had been another way for her to take? An injection administered by a doctor she trusted, or pills brought to her by Dad or one of us? I'm not sure. Perhaps she wouldn't have wanted anyone else to bear the responsibility of having ended her life. Also, I think part of her relished the challenge, the control and the independence of doing it the hard way.

In the obituary that was read at Mum's funeral, Vicky remembered a saying that Mum was fond of quoting: "Old age is not for sissies." Mum was no sissy – as Emma put it later, Mum was a soldier. She embarked on her final journey with steely determination, astonishing courage and immense dignity. I feel honoured to have been a part of the process. I've seen that it is possible to choose and achieve a good death, on one's own terms. I used to be in favour of changing the law on end-of-life choices; now I am evangelical about the importance of autonomy in terminal illness and death. I'm so proud of what Mum did, and of how my father, my sisters and I helped her to achieve it. But that doesn't alter my passionate belief that people in Mum's position, or Tony Nicklinson's, or Martin's, should have more, better and easier choices.