I finished the third book of the series by torchlight under the doona, desperate for more. But there was a problem; the next book had yet to be written.

By the time it came out, I would be halfway through high school. The wait seemed like an eternity.

Isobelle Carmody was 14 years old when she began writing The Obernewtyn Chronicles. Her father had been killed months earlier by a drunk driver. Her mother worked to support eight children. Carmody, the eldest, was left in charge, keeping her siblings in line by telling stories. There was little time for homework and she was too weird to have friends.

At school in Geelong, she learnt about the bombings of Nagasaki and Hiroshima, and the Manhattan Project, which produced the world's first nuclear weapons.

She began to create a world where humans eked out an existence between vast tracts of poisoned earth. Anyone different was despised as a mutation born of the holocaust – none more so than those who could use their minds to communicate with animals and humans, to see the past and future, move objects, coerce the unsuspecting, and send and receive emotions.

"I love going into that world," says Carmody, 43 years on. "It's been a constant place in my life that I could go, which I can't go to any more."

Carmody is sitting opposite me in a cafe on a sweltering Sydney day. She is 57. I am 31. I feel, probably foolishly, as though we have come a long way together.

When I started reading The Obernewtyn Chronicles, I had never used the internet, though it existed in fledgling form. Today, on social media and sites devoted to the series, fans are celebrating the release of the seventh and final book, The Red Queen. They are all excitement, disbelief and capital letters.

But what of the author? What does it mean to have finished the story she began so long ago?

"In the final stages of writing, people kept saying to me, 'How do you feel?' " she says, in a broad-ish Australian accent that somehow seems at odds with her stories of noble quests through snow-capped mountains.

"It was as if I was ill and recovering ... 'This must be so huge for you. You'll crumble at the end of it.' But I don't feel like this vapoured, delicate creature who writes. I'm not that person."

Nor does she look it. Despite the heat, Carmody wants to be photographed in her thigh-length scarlet coat. There are red crystals dangling from her earlobes. She talks fast and tells stories rich with detail.

The house where she finished The Red Queen was a holiday place north of her home in Brisbane. She had told her partner, the Czech musician and poet Jan Stolba, and their 17-year-old daughter Adelaide that she would not be back until the book was done.

She allowed herself one trip outside each day, walking across the road, stopping short of the beach and eating cereal for lunch as she watched the ocean.

Everything about the last day of writing is sharp in her mind. The tremor that shook the earth just as she finished the final chapter (yes, really); the white dog that ran up the steps from the beach as she left the house; the train station where she waited on the final leg of her journey home. "There was nobody there but me," she says. "It was just beautiful."

Suddenly, she is weeping in the cafe, just as she did for about an hour at the station. "Sorry," she says and pours herself some tea. "It was huge to end it – much bigger than I realised it would be."

For decades the Obernewtyn books have inspired intense reactions. There are hundreds of letters from readers, some warm and grateful, others so impatient and acrimonious that Carmody's publisher has never let her read them.

Originally, Carmody thought her series would be a trilogy. By book five, she was apologising in the dedication. "A special heartfelt thank you to the faithful readers of this series for waiting, patiently and impatiently, for what turned out to be, after all, not quite the last book."

"I was regretful many times over the years that I ever said how many books there would be," she says today. "People kept acting like I was padding it. That was never the case. I was just trying to get to the end."

From time to time, readers complained when she published other books. For all the mutterings about the Obernewtyn drip-feed, Carmody is prolific. She has written four other series (in varying states of completion), stand-alone novels (seven of them), picture books (some illustrated by Carmody) and dozens of short stories. She is working on a PhD and a screenplay of her novel Greylands.

When she was a new author with what looked like a hit series on her hands, there was pressure to turn immediately to the next instalment. But there were other ideas she wanted to explore; she wrote Scatterings (1991), another post-apocalyptic tale, and The Gathering (1993), a dark and gritty fable set in a seaside town. Both won prestigious awards. The Gathering, in particular, was critically acclaimed.

"I think at that moment, [the publishers] stepped back and said, 'Well, wait a moment, maybe we should let her do what she wants'," Carmody says. She swore never again to work on sequels back-to-back. "I can't just write like a sausage machine."

Deadlines have always been difficult for Carmody; in the weeks before our interview, it becomes clear the publisher is scrambling to print The Red Queen on time. Does the last-minute rush ever affect the books? "Yeah, there's no question," she says. Early editions of later Obernewtyn books have mistakes and inconsistencies. Carmody fears there may be errors in The Red Queen too.

Had there been more time, she might have edited the 1108-page monster into something slightly shorter. "But no one wanted to wait that long," she says. "I also believe that you think better when you are under pressure ... In the final stages of this book, I was under terrible pressure, but it also felt like I was mustering up to a heroic effort. It felt right somehow."

It is not unusual for readers to clamour for the next instalment of a beloved series. Carmody met American writer George R. R. Martin at a literary festival years before the television series Game of Thrones brought his ongoing fantasy saga international fame. More recently, the incessant demands of Martin's fans prompted a passionate reprimand from fellow writer Neil Gaiman: "George R. R. Martin is not your bitch".

The only way to take all this impatience, says Carmody, is as a compliment. What even the most aggressive reader is really saying is that her books mean something to them.

Carmody had not yet finished The Red Queen when she received an email from the mother of a 16-year-old fan. Her daughter had a rare form of bone cancer. She was dying. One day, the mother found her child weeping in her bedroom. She was thinking of all the things she would never experience. One of them was the end of The Obernewtyn Chronicles.

Carmody began to read her unfinished manuscript aloud over Skype. The child would eat dinner – tiny, gourmet feasts carefully prepared by her mother – and listen until she fell asleep or felt too sick to go on.

They never reached the end. The Red Queen is dedicated to Tash's memory and there is a character named after her.

Of the many readers Carmody has met, some have made lasting impressions. The young woman who established the fan site obernewtyn.net has become a close friend. Another has proved a sharp-eyed editor for Carmody's unpublished books. Many have said they feel that the conclusion of The Obernewtyn Chronicles marks the end of their childhood.

For me, it has been a long time since I had a whole weekend with nothing to do but read. I still love science fiction and fantasy, but these days it is harder to dive into anything with the single-minded passion of the 13-year-old who fell in love with Carmody's story.

It has never been quite the same since that first sleepless weekend, but there are moments in the final book – a key revelation, an unexpected kiss, a daring escape – that twist my insides with joy.

The Red Queen is published by Viking at $32.99.

ANOTHER THING

Isobelle Carmody is planning another series, The Beforetime Chronicles, set in the world of The Obernewtyn Chronicles before the holocaust. "I could only bear to end, knowing I could have that," she says.