The letters, mostly from strangers, had begun arriving just days after her baby daughter Azaria was taken by a dingo while on a camping trip at Uluru in August 1980. The stream continued steadily throughout her trial for Azaria's murder, her wrongful conviction, three years in prison, the subsequent Royal Commission and several coronial inquests.

In them, the public sympathised and excoriated, prayed and threatened. They told her of their own lost children and family dramas, or kept her up-to-date on life on the outside. From daily minutiae such as milk prices, births and deaths and political events, to marriage proposals, declarations of support or ugly abuse, the public response to Chamberlain-Creighton's plight was a remarkable snapshot of life in 1980s Australia.

And today, 36 years after her daughter's death, the letters – more than 20,000 of them, along with thousands of emails – keep on coming. But instead of piling up around the house, they're now meticulously filed in colour-coded folders at the National Library in Canberra, where they fill nearly 200 boxes. Chamberlain-Creighton made the decision to give them to the library once their value became obvious, and devised a filing system that developed piecemeal over the years, as she learned how many researchers, from how many different sectors, were using the letters.

Chamberlain-Creighton herself is still intrigued by the letters and by her own presence in the lives of so many people she is never likely to meet.

"It fascinated me that all the men in one family would write, and in another one it was all the ladies that were writers. It seemed like it ran in families," she says.

"I had one family where there was a father, two uncles and then three brothers, the whole six of them wrote to me over the years, and more than once. The interesting thing was, often they didn't know that each other was writing."

She vividly remembers a woman rushing up to her in the street one day, years after the Royal Commission, and giving her a hug, before launching into a series of updates on people she'd never met. There was an awkward moment before Chamberlain-Creighton understood that this was one of the many people who had written to her over the years, and had come to think of her as family.

By the time playwright Alana Valentine got wind of the letters in the library, it was clear that the 1980s and early 1990s had become the most documented period in Australian history, thanks to the files that have become known as Lindy's Letters.

"I thought that it would contain the full panoply of human nature in there, because it roused so many different kinds of emotions," she says.

"I had a sort of a nascent instinct that this would be a community, only in archive boxes instead of in a geographical place."

Valentine has made a name for herself through her plays that dramatise events in an eclectic range of Australian communities – football fans, train crash survivors, former prison inmates, gay men in remote Australia. What attracted her to Chamberlain-Creighton's letters was the chance to read the words of ordinary people who weren't necessarily known for being literate or articulate.

"What I really wanted to see was what people do reach for in those moments of confusion and helplessness," she says.

"How do they put in words to a stranger what it is that they want to say? And I certainly found that in abundance."

The resulting play, Letters to Lindy' produced by the Merrigong Theatre Company in Wollongong, is as much about the loss of a child as it is about a sensational trial – an ordinary family thrust into an unimaginably harsh spotlight, and the people who responded to it.

"No play is about information, and no matter how many colourful characters you find in this collection, neither is it just about the characters," says Valentine.

"It has to be about a relationship, and this play is about not telling Lindy's story in the same way that it's been told in the books and the films, but telling of her relationship to this collection."

Artistic director of Merrigong Theatre Company Simon Hinton says the play, while based on the words of the letter writers and of Chamberlain-Creighton herself, tells a larger story about Australia.

"I like describing Alana as a truth-telling playwright," he says.

"There's a deep truth underlying the work, not only about Lindy as a mother, but also about Australia, about our society, and a range of really interesting topics – what we demand and expect of women in the public eye, all of our prejudices and those things."

Even the way the letters have been catalogued tell their own story. Entire families sat down to pour their hearts out to Chamberlain-Creighton after her baby was killed, and kept on doing it long after she was exonerated, divorced, remarried and living an ostensibly normal life again.

But there is nothing normal about a woman so clear-headed that she sees value even in 20-page screeds from a bored housewife detailing, quite literally, a conversation with her husband about what to have for dinner. Chamberlain-Creighton has added a precis of contents to thousands of the letters, to make it easier for researchers to find what they're looking for. She's even devised a star system to mark extreme versions of certain types of letters. Reading them is, says Valentine, "like having Lindy sitting on your shoulder".

Surprisingly, given the bitter controversy of the legal case, Chamberlain-Creighton has received relatively few negative letters over the years – she puts it at about 3 per cent – and maintains she treats them as comic relief.

"Seriously, support is beautiful and lovely, but it gets to a point where you are so soaked in it that anything's a change, and it's so out of left field, it's hilarious!" she says.

She remembers one vicious letter from a woman who told her Azaria was better off without her. "I wouldn't waste my time writing to you," the letter said, and enclosed was a single black bootie. Chamberlain-Creighton still screeches with laughter at the thought of the twisted logic of somebody penning such a missive. Even funnier, she says, was that years later, the actor Meryl Streep, who played her in the film Evil Angels in 1988, received an identical letter and another black bootie after the film came out. Streep duly passed on the missive.

"Now I've got a matching pair to go with the dress with the red ribbon, so I put red ribbons in them, and put them in the cupboard and thought thanks for the gift!" she says, still hooting with laughter.

She still receives about1000 emails a year, many of them from young people who were born long after the trial. Many, she says "apologise for their parents' behaviour, which fascinates me, saying, 'We lived through it and they were just so caught up in it for so long'."

"It's amazing the number of kids who say, 'I've studied it for myself now, and you're my hero'. And to be the hero of teenage girls is amazing, let alone teenage boys."

For Jeanette Cronin, who plays Chamberlain-Creighton in Letters to Lindy, it's this stranger-than-fiction outlook that has made the role such a gift – quite apart from the line-up of stars who have played the character before her.

"It instructs you, like all great characters and writing. If you listen to it, it instructs you how to play it," she says.

"Lindy is a gift, and she's a very bright, logical, practical person, and she's a chronicler. We depend on chroniclers in the world."

Valentine, too, says a new Chamberlain-Creighton emerges from the letters, and through the play – one the public might think they already know, but don't.

"My play is about why she's done this and what this archive represents to her," she says.

"I think that once people see the kinds of letters, they'll start to understand what the relationship Lindy had with them is and how that relates to her grieving, her incredible ability to forgive and to let go."

Letters to Lindy is showing at the Illawarra Performing Arts Centre July 26-August 6, the Canberra Playhouse from August 9-13, the Seymour Centre in Sydney from September 2 and the Shoalhaven Entertainment Centre on August 30.