She couldn't find any page in the book that didn't have something she wanted to alter, be it a comma, one word, a sentence or even whole paragraphs. "Eeugh!" she exclaims, burying her head in her hands at the recollection.
Yet The Rehearsal, published in 2009 when Catton was 21, has been translated into 12 languages, won the Betty Trask Award, was shortlisted for the Guardian First Book Award and the Dylan Thomas Prize, as well as being longlisted for the prize formerly known as Orange. The novel marked one of the most lauded literary debuts in decades and Catton became the glittering "golden girl of fiction".
Her audacious, extraordinarily accomplished book "represents a starburst of talent and the arrival of an author wholly different from anyone else writing today," enthused the Sunday Times reviewer, while Kate Atkinson hailed "a truly exciting new writer". The Rehearsal, she said, is "compulsively good," calling into question "the very nature of truth itself".
Now, Canadian-born, New Zealand-raised Catton - who will be 28 this month - is back with her second novel, the Man Booker Prize longlisted The Luminaries. If The Rehearsal is, in the words of American writer Joshua Ferris, "a mesmerising, labyrinthine, intricately patterned and astonishingly original novel," then The Luminaries is all of that and more, much more. It is deliriously haunting, compellingly written and utterly addictive.
"The way it's written is so completely different from The Rehearsal," says Catton. "I never want to write the same book twice anyway, because people often do the same thing over and over again in literary fiction. For me, the novel is infinitely malleable - you can do anything with it and be as playful as you choose."
A history mystery, The Luminaries is certainly playful; it's a sensational, postmodern, post-Jungian pastiche of a Victorian sensation novel in the mould of Wilkie Collins's The Woman In White, say, with subversive shades of Agatha Christie. It made Catton the bookmakers' favourite to win this year's Booker long before the shortlist is revealed on Tuesday.
At 832-pages, it's a breeze block of a novel. People josh about sprained wrists after simply picking it up let alone reading it. Her publishers are working on ways to make it lighter in paperback. Had it been any longer, the hardback would have succumbed to spinal injury, she says when we meet the morning after her Edinburgh International Book Festival session. We drink coffee in the restaurant at the Royal Botanic Gardens - a venue chosen by her publicist because it's the nearest the capital can approximate to the landscape of the west coast of New Zealand's South Island, where The Luminaries is set during the gold rush year of 1866.
Readers of Rose Tremain's 2003 novel, The Colour, will recognise the territory, although it's been familiar to Catton since her childhood. Her mother designed and managed school library collections on the west coast for New Zealand's National Library and often took her daughter there with her. "I also did a cycling trip there with my dad. I remember thinking a lot about the gold rush and being enthralled by it," she says.
The Luminaries begins on the stormy night of January 27. A young fortune-hunting lawyer from Edinburgh, Walter Moody - a man who "was always in some chamber of his mind perceiving himself from the exterior" - fresh from the terrors of his voyage to New Zealand, walks into a half-finished, shabby hotel in the rough frontier town of Hokitika.
He stumbles into the smoking room where 12 men are meeting in secret to investigate a series of unsolved crimes involving a dead drunk, an opium-eaters' den, the apparent attempted suicide of a whore, a ghostly ship, a bloody coffin, a missing cache of gold and a young man's mysterious disappearance. Moody finds himself in a world of "rolling time and shifting places," a "small, stilled world of horror and unease," a world of role-playing. "One should never take another man's truth for one's own," declares Moody - and he should know.
The unique narrative twist Catton employs, however, is the use of astrological star charts, rotating 12 characters born under 12 different star signs - eight other characters make up the cast of 20. It sounds ridiculously tricksy but her trick is to have pulled it off to luminous effect. The device is derived from the geometrical idea of the golden spiral. Although she has no truck with newspaper astrology, Catton - a Libran - became so fascinated by the notion that she asked herself whether she could write a book in 12 parts, where every part stands in golden ratio mathematical relationship in terms of length to the part that comes after it. When she did the arithmetic, she discovered that it would be more than 300,000 words.
She parked the idea, then began to wonder whether she could still write a novel in a dozen parts but in which each was half the length of the preceding part. This time the equation came in with a word count in the high 200,000s. So, the final chapter is only 65 words long, half the length of the chapter heading. (Throughout she uses precised headings in the style of Dickens - though he's not an influence - and George Eliot, who is.)
Catton is the youngest of three children. Her father is American, a philosophy teacher. When she was six years old, the family moved from Canada to Christchurch, New Zealand - her librarian mother's a Kiwi - following a year's stay with relatives in Leeds when her father took a sabbatical. A bookworm from an early age, she began writing when she was about five.
A teacher of creative writing at the Manukau Institute of Technology Faculty of Creative Arts, Catton is a joy to talk to, fizzing with ideas despite feeling jet-lagged. She shares her life with American poet Steven Toussaint, who reads and waits patiently for her as we talk. He's always her first reader and it's like having an in-house editor at home in Auckland, she confides. Do they argue if he suggests a rewrite?
"Oh yes!" she exclaims. "When I killed off a character in The Luminaries he had lots of critical things to say about how I'd handled it. There were tears on both sides. The following morning I woke up and made the changes he'd suggested because, guess what, he was right."
Clearly, as she writes in her Acknowledgements, he's been there "for every conjunction, every opposition and every dawn"?
"He has indeed," she replies. "But I'm so glad that Steven's a poet. I don't think I could bear to live with anyone who writes fiction. Two novelists in the same house, that would be madness."