I was just 14 years old when I first wrote to John Marsden. It was 1997 and I don’t now remember, precisely, the content of my letter (in my unreliable mind it was at least six A4 pages, handwritten and scrawling), but his neatly typed reply hints at what it contained. “Dear Monica,” it begins. “Firstly, I’d better assure you that you’re not getting the same letter as everybody else! I don’t do that to anyone.”

Some 18 years later and I’m reading aloud to the Australian writer the letter he once sent me. “It’s true, I don’t send the same letter to two people,” he says.

As I run through the rest of his letter, in which he patiently answers my queries about his guerrilla teen series Tomorrow, When the War Began and another of his young adult (YA) novels, Letters from the Inside, he admits: “They’re my stock answers to those questions. Worded differently each time but I can only give the same answer when I’m asked the same question!”

This hurts, just a little. I hope this time around my questions are a little more original.

When I first approached him for this interview, he did try to find my letter. It turns out the 64-year-old author doesn’t discard any, and has accumulated six filing cabinets of haphazardly organised mail over the decades. “I don’t like to throw things out, but I’m not exactly a hoarder,” says Marsden. “I like things to be complete. I’ve got what I privately call my ‘completion complex’.”

Marsden is also – and I’m a little wary about writing this, considering its consequence – fairly meticulous about replying. And no, not a headshot with “xo JM” hastily signed in the corner; we’re talking individually crafted letters. This, despite the fact that 90% of the letters he receives are “stock standard”. “The worst,” he says, are those which teachers have ordered students to write. “The incredible banality of those letters is just depressing to the spirit.”

Every now and then, however, he receives a letter that stays with him. In one school batch, a student had written in the format set by her teacher but had included another sheet which read: “The letter you’re getting attached is the one that the teacher made us write, but this is what I really want to say.” Marsden describes that secretly stashed letter as “full of life and energy and personal voice. It was so refreshing to find it and realise there was this rebellious spirit in that class.”

Another memorable exchange came from a Sydney teenager. It dated back to the 1980s and went on for a decade. Over the course of the pen-pal relationship, the girl was diagnosed with bipolar disorder and sent Marsden one of her paintings – a portrait of a young woman, painted in bright colours but in fragments – which he describes as “absolutely stunning”. At some point she wrote to say she’d destroyed all her art. And over time the correspondence petered out.

Marsden quotes Gertrude Stein, who supposedly told the painter Henri Matisse: “There is nothing within you that fights itself.” In contrast, Marsden says the teenager’s painting, which still hangs on his office wall, is “very much the work of someone who’s got forces within fighting herself”.

You could easily characterise Marsden’s own writing this way. “You’ve got to have that inner conflict in art or literature to make it really memorable and profound and powerful.” Marsden considers his stories “uplifting” and ultimately about the “nobility you can find in ordinary people”. But love, friendship and kindness are usually thin planes of light running through big sweeps of despair (war-torn Australia, colonialism, child abuse).

With more than 5 million books sold internationally, it’s no surprise he has many young fans turn to him, with some intensity. Marsden says his own lifelong experience with depression has perhaps made him more sympathetic.

He says some such fans pour their heart out to him in long, despairing letters, and beseech him to reply, but make one critical error: they forget to leave a return address. The author seems a little haunted by these. “That’s the trouble with having an over-vivid imagination. You sort of imagine them sitting by the letterbox waiting for the letter that never comes and thinking ‘betrayed again, yet another person who’s let me down’.” He advises me not to dwell on this too long.

Alongside his literary pursuits, Marsden has been equally passionate about education, and after a career as a teacher established his own alternative school, Candlebark, in the Macedon ranges of Victoria. He has described the school as “somewhere between Steiner and The Simpsons” and has plans to open a secondary school in the area, focused on the arts.

He recently became a father for the first time, after marrying Kris, a Candlebark parent with six sons, aged 10 to 20, from her previous marriage. Marsden says the younger ones in particular are avid readers, and while they tend to avoid his books (“it’s too awkward”), he’s able to share with them a love of JK Rowling, and likens the young adult fiction giant to Charles Dickens in her ability to create memorable minor characters. “Snape is one of the great creations of English literature.”

When I ask for Marsden’s thoughts on the current crop of violence-filled dystopian YA novels, like The Hunger Games, The Maze Runner and Divergent, he avoids commenting on them directly but later discusses what he sees as a major growth in “shock-horror” novels that are “written with a template”.

“They’re underpinned by the belief that the more shocking and savage the book the more likely it is to sell to teenagers. So I think it’s quite a cynical, commercial push – not coming from publishers but from writers themselves, who want to be rich and famous.”

It’s been seven years since Marsden last wrote a YA novel, and as the stage adaption of his classic children’s novel The Rabbits, illustrated by Shaun Tan, premieres on Friday at Perth festival, the writer has turned his attention to a new demographic.

His first adult novel, South of Darkness, released in 2014 and set in the late 1700s, centres on a 13-year-old orphan boy in London, who decides his fortunes lie in Botany Bay. The work seeks to unpin the mythologies that blanket both colonial and Indigenous Australia, telling a story filled with vibrant character interactions and historical detail.

While I may never discover exactly what I wrote Marsden in that teenage fan letter, I do recall pouring my heart out, gushing about how intimately I understood his characters – but also urging him to finish his Tomorrow series, quickly, because I sensed I was soon going to grow out of them (it happened – I never read the final book).

Perhaps it was in the spirit of such frankness (or just an old teacher’s habit) that in his reply he saw fit to point out one of my spelling mistakes. “It’s interesting that you remember the spelling comment and forget the compliments,” he says. Oh, I remember the compliments too, I say. In fact, with the warping effect of time, both had grown larger. “According to John Marsden I’m a great writer and a terrible speller” – so went my personal mythology as the decades passed.

Because you’re probably wondering, Marsden does not have a Twitter account but he does have an email address. But he removed it from his website: “I felt obliged to answer each one I received, and although many of them were wonderful and moving and memorable, I soon found myself doing nothing from 7pm to midnight, every night, seven days a week, but answering emails.”

There’s always snail mail.

• The Rabbits is playing at Perth festival 2015 until 17 February. South of Darkness is out now.