In Guardian Australia’s new series Ask a Children’s Author, we’re palming off the hard work of journalism to readers – and asking them to interview their favourite authors for us.

First up we had Andy Griffiths, author of the Treehouse and Bad Book series, who answered all manner of questions: about how to write, about his next book, and about whether or not his bum has come off.

For our second edition, we’re appealing to older readers: fans of Garth Nix’s bestselling fantasy books, including the Old Kingdom, Seventh Tower and Keys to the Kingdom series. Nix writes for “children, young adults and adults – sometimes all at the same time”, he reminds us. “You’re never too old to read any of my books.”

You’re never too old to ask him a question, either. So if you or your child has a burning query for Nix, please film it and email the video to australia.culture@theguardian.com with the subject heading “Garth Nix” – or you can just write the question into the comments below. (Please include the question-asker’s name and city and, if you like, age.)

The question might make it into the one we publish with all his answers – so long as you send it before Monday 25 May …