To start with the question millions of children want answered: no, Andy Griffiths does not live in a treehouse. His very nice home in Williamstown, Victoria – firmly on the ground – does not contain a mud-wrestling arena or anti-gravity chamber, but it does have a very sleek front room with big art and huge windows looking out over the ocean. Griffiths – softly spoken, angular from years of running, tattooed from years in punk music – walks me through his back yard, to a little granny flat on stilts (almost a treehouse). This is where he dreams up books that have made him Australia’s fastest and highest-selling author: 19m copies sold around the world, 13m of those in Australia. “This is where I have my fun,” he says, discreetly nudging a rubber lizard out of my path with his shoe. Inside is a child’s dream: a grotto of knick-knacks, practical jokes, junk and toys. There’s a jar labelled “Andy’s vomit”; a crocodile head with plastic babies spilling out of its toothy grin; a section just for banana-shaped things called “Banarnia”; and a lonely fake poo, lurking on a bottom shelf. In the corner is a six-foot tall model of the ever-changing, multi-level treehouse created by Griffiths and his longtime illustrator Terry Denton in their bestselling book series, which is finally coming to an end (more on that later). It was Griffiths’ oddball Just series, after an annoying boy called Andy, that made him famous in Australia in the 1990s – but it was the Treehouse books, starting with The 13-Storey Treehouse and now ending with The 169-Storey Treehouse, that made him a global bestseller. “The Just books are 80% anarchy, 20% nice. Treehouse is 20% anarchy, 80% nice,” he says, to explain the latter’s popularity. “It is much more conventional – most people don’t need a world of punk rock chaos and gratuitous horribleness, as amusing as I find it.” The 61-year-old is a big kid, an ex-punk rocker and a former teacher rolled into one, all of which has shaped his view of books as a final frontier: a place of anarchy and danger where children have permission to imagine the wildest, funniest, scariest things they can conceive. “Books are a dress rehearsal for your biggest fears,” he says. This tends to make adults nervous. In 2004, some schools and bookshops refused to stock The Bad Book: a collection of ditties that included a grandma-eating poo and a mother sending her child into traffic. In response to the handwringing, Griffiths wrote an open letter arguing that the book was not intended to “encourage young people to be better”. “Terry and I believe that children’s book writers have one overwhelming duty: to fire the reader’s imagination and encourage a love of reading,” he wrote. “All other purposes are secondary.” Sign up for the fun stuff with our rundown of must-reads, pop culture and tips for the weekend, every Saturday morning His publisher was more direct: “Adults are reading it with an adult mind, but the kids know not to set fire to their willies.” A big part of his career, he says, has been “settling adults down. They’ll be watching me at school events tell the kids to annoy the teachers, to swing on the clothesline to fly off into outer space, and the children are laughing uproariously. But afterwards, the kids all want to read books – and that’s when the adults get it. It’s OK to imagine things. I was always proud when kids told me their school had banned my books – ‘yes, I’m absolutely on the right track!’” One of his earliest memories is reading Der Struwwelpeter, a collection of brutal German morality tales. In one, a girl is burned alive after playing with matches; in another, a boy starves to death after refusing to eat his dinner. It is horrifying – and Griffiths loved it. “I never recovered from that combination of horror and humour – it’s what I’m trying to do with my readers all the time.” During his time as an English teacher in the late 1980s, he says, “the kids were very suspicious if I took them to the library: ‘Ugh do I have to be here? Books are boring!’ I realised they hadn’t had the huge advantage I’d had, growing up in a house with books like Dr Seuss, Enid Blyton, Ray Bradbury – books that were fun to be in.” One day, he asked the children: did I ever tell you about the day my bum ran away? He instructed them all to write. “None of them wanted to be caught with the most dull book, so they went to town. We wrote things, read them, criticised and edited them. I still listen really carefully to the reactions from kids when a new book comes out.” Inspired by his time in punk bands, Griffiths began making little zines of pranks that he would sell to kids for 20 cents each. The first was called Just Tricking. “I’d leave with a big bag of 20 cent pieces and spend the night assembling more,” he says. When he looked for a publisher, none would bite – but one educational publisher saw potential in his stories as a creative writing textbook aimed at teachers. Published in 1993, the textbook was illustrated by Terry Denton. When writer and artist met for the first time, both men saw a streak of madness in the other that would fuel the next 30 years of work. It was Denton’s promise that he’d illustrate whatever Griffiths wrote that finally got a publisher on board; and it was that publisher who introduced Griffiths to his future wife, Jill, who has edited all of his books since the first, Just Tricking in 1997. “She can be totally honest with me in a way she can’t be with any other author. She worries about their feelings,” he says. For Just Tricking, Denton scribbled tiny unrelated gags in the margins and flickbook illustrations on each corner to keep the bookphobic kids interested. “I told Terry … don’t make it look like a normal book – we need to trick them into enjoying it,” Griffith says. “And as we went along, I realised what an amazing thing he was doing.” Sure enough, Just Tricking, Just Stupid, Just Disgusting and all the sequels, were just enormous. His next series was about a boy who wakes up to find his bum has run away to take over the world. “Outrageously stupid,” says Griffiths, of The Day My Bum Went Psycho, which spawned the sequels Zombie Bums of Uranus and Bumageddon. “I’m really proud of them.” They gave him his reputation for being “the bum-poo-fart author”, as he puts it. “When you’ve explored the limits of disgusting, horrible humour, the most shocking thing you could do is a book that’s not disgusting or horrible. So we eventually got to Treehouse.” The Treehouse are noticeably sweeter than anything else Griffiths has written; as he puts it: “The Just books are punk rock, Treehouse is new wave.” The 13-book series follows Andy, Terry and Jill on their adventures in their treehouse home, which gains 13 levels every book. The premise was in part inspired by conversations Griffiths would have with children: “They’d say stuff like, ‘Do you realise that Terry drew you being squashed by a grand piano [in a book]? Are you mad about that?’ And I’d go, ‘Did he?’ ‘Yeah, it’s on page 67.’ I’d go, ‘Right, when I get home, I’m gonna smash a grand piano over his head.’ They really loved the idea we lived together. “I saw Treehouse as a kind of fake biography of ourselves, arguing and trying to write books. And when Terry feeds his underpants to the sharks, Jill’s the one who comes in to perform open shark surgery, which is a metaphor for how she fixes our stories when we get all tangled up or go too obscure.” Some might be surprised that such a popular series with infinite potential is ending, but Griffiths says it has reached its natural conclusion. “I’ve never wanted to get to the point where the reader says, he’s phoning it in. That’s my biggest fear,” he says. “It was getting difficult to write a plot that didn’t repeat some other aspect of what we’d already done.” Griffiths and Denton are assembling a guide to all 169 levels of the Treehouse and after that, “something is suggesting itself to me”, he says cryptically. Has he ever considered writing for adults? Will he now? “Initially, I wondered if I would get ever ‘serious’, but I’m incapable of it,” he sighs, smiling. “I wanted to be a serious writer and yet I can only write nonsense – but jeez, everyone seems to like it!” And given his young fans often need their books read to them, in a way he is writing for adults too. “Writing a book that a kid can’t read makes almost no sense to me. If I can entertain everyone – well, that’s the best of all worlds.”