It’s October 2015 when the Sydney journalist Nikki Gemmell gets a call from the police. Her mother has died under suspicious circumstances. Can she come down to the morgue and identify her body?

In the shock of grief, there is another reckoning. Did her mother take her own life? And if so, why?

She had potentially decades to live, says Gemmell. She had grandchildren to enjoy and a job she loved at the New South Wales Parliament House. Yet she was found dead in her flat by builders, having swallowed pills and with a book by Philip Nitschke by her side.

In the months before her death, Gemmell’s mother, Elayn, had gone from a carefully groomed, dynamic, independent woman to someone dependent on drugs and planning to end her life.

Gemmell’s account of this strange, confusing and distressing time is the subject of her new book, After. But After is also the story of mothers and daughters.

Gemmell’s generation – she is 50 – came of age in an ostensibly post-feminist world: she grew up believing she could do and be anything she wanted. Her mother, however, missed the sexual revolution and the second wave of feminism only by a few years. Denied the early freedoms of those born a decade or two later, Gemmell’s mother had three children before she bolted from marriage and suburbia, and set off for Sydney for a career and the single life. She even changed the spelling of her name along the way from Elaine to Elayn – the new spelling reminiscent of the word élan, synonymous with flair and vibrancy.

“She would have been the magnificent single, childless woman – she would have seized life,” Gemmell tells Guardian Australia. “All the opportunities came too late.”

Instead, Elayn’s frustration with missed opportunities bubbled over into her relationship with her highly successful daughter – now a columnist at the Weekend Australian and the author of 17 books, including the bestselling novel The Bride Stripped Bare. “You’re accelerating into the future, they’re in the past – they’re stagnating and you don’t have that curiosity about their lives and they feel it acutely,” Gemmell says.

Gemmell records the decades of tension in After: Elayn cringing when Nikki speaks proudly of the family’s history working in the pits around Wollongong. Elayn telling Nikki that she wished she (Elayn) had never had children. Elayn disapproving when Nikki gets glasses, and calling her ugly. Elayn not spending any time with her daughter during her first pregnancy. Throughout the book, this tension between the pair sparks recognition from anyone familiar with those family dynamics: all the small slights and points of pride; the issue of who is going to back down first; the huge amounts of love that are dammed up while approval is withheld; and finally, the release of tenderness at the end when it’s almost too late.

If this were just a book about the treacherous and tender currents that run between mothers and daughters, it would be enough. Instead, After also explores euthanasia, the police investigation that followed Elayn’s death, and the increasing problem of elder opioid abuse. How does a woman with no history of drug use suddenly spend weeks blissed and – as she tells her daughter – “bombed out on drugs”? And how did she access the pills?

Elayn, who’d never touched drugs in her younger years, became something of an accomplished doctor-hopper, says Gemmell. After a foot operation went wrong, she was prescribed opioids for chronic pain. Her initial use quickly became a dependency and, as her body became more immune, “she went from doctor to doctor in different suburbs, looking for prescriptions”.

The drugs demotivated and isolated Elayn. “She was very glamorous,” Gemmell says. “She used to take an hour each morning to put on makeup – then she retreated into this very claustrophobic and lonely world. I was so shocked when I walked in [during those final weeks]. There was just piles of paperwork and stuff, things she wanted to throw out but hadn’t gone past the front door with them.”

Elayn was also reluctant to seek help from her children, suspecting – wrongly, says Gemmell – that they might have placed her in a nursing home.

An autopsy discovered the cause of Elayn’s death was multi-drug toxicity – basically, a drug overdose. By the time she died, Elayn had been communicating with the euthanasia advocate Exit for around eight years.

“Her death was a shock to a lot of people,” says Gemmell. “Most people didn’t know about the euthanasia. There was a sudden, rapid shrinkage into a world of pain and despair. And she saw no way out of it.”

While Elayn was experiencing pain, Gemmell says she “didn’t engage with mindfulness, physio, pain clinics – she was demotivated because of the opioids. It was amazing she could get all these drugs. There were no checks and balances.”

Gemmell first wrote about her mother’s death in her column in the Weekend Australian. Afterwards, she said, “The haters used to write in and say; ‘your mother died a junkie’ – but I can’t bring myself to think that ... She was pain-free, she was happy.”

Investigating her mother’s death led Gemmell to connect with Philip Nitschke and other euthanasia activists – “Exit listened to her and we didn’t, as a family. It is a complete community,” Gemmell says. At her speaking events for the book, she says she encounters a huge hunger for discussion about euthanasia.

Gemmell does not view her mother’s decision with bitterness but rather appreciates and wonders at the complexity of her life.

“It amazes me, the courage and resourcefulness of this elderly woman racked with pain. She managed to do it – she picked her moment with builders in the flat because none of us [her children] would find her.

“When I think about it – all those withholdings, all those wasted energies, all those cruelties, all those hurts … we had both reached a level, though, where we wanted to put decades of mind-fuck behind us. And it was great, the final year – and I wonder if that helped her to release herself.”

• Crisis support services can be reached 24 hours a day: Lifeline 13 11 14; Suicide Call Back Service 1300 659 467; Kids Helpline 1800 55 1800; MensLine Australia1300 78 99 78

• After by Nikki Gemmell is published by 4th Estate and available now. Nikki Gemmell is appearing at Sydney writers’ festival in May 2017