Yes, she's just six novels - and three children's books - into her career and Moriarty has had the rights of three of them optioned.
It was with the former advertising copywriter's fifth novel, The Husband's Secret, about a woman who finds a mysterious letter from her husband "to be opened only in the event of my death", that Moriarty really hit the big time, selling more than a million copies in the United States alone to date. But, she says, despite her success, life goes on as before.
"It doesn't change your day-to-day life - I'm just getting ready to write the next book. In a way, I just have to put all that aside. It's all exciting, but it doesn't actually change the way I do my job. Except I do feel, I guess, with [my next] novel the weight of expectations for the first time because Big Little Lies debuted at No. 1 on the New York Times bestseller list."
Set in a fictional part of Sydney called Pirriwee, which Moriarty says is loosely based on the city's northern beaches, the novel starts at the end, with the death at the trivia night, then works its way forwards towards the main event. The suspenseful page-turner keeps the reader guessing, right until the last possible moment.
The main players are three school mums: beautiful, rich Celeste, witty, confrontational Madeline and timid solo mother Jane. Through the trio's friendship and interaction with the other mums at the school, Moriarty explores the consequences of what can happen when bullies - at school, at home and in social situations via gossip and bitchiness - run riot and secrets are allowed to fester.
The book also delves into heavy themes of domestic and sexual violence, but Moriarty's wry touches of humour save it from becoming too heavy.
"I think that's the trick with writing a book like that. I just started reading a book where there's one particular character so horrible and so distressing that you think, I just don't want to read this anymore. It's hard because you need your villains in a book but you also need to make it a pleasure to read," she says.
One of Moriarty's other writing strengths is her characterisation - almost every reader would recognise the traits of the Blonde Bobs, the mothers who run the schools' parents associations like Mum Prefects who all have the same haircuts. "They feel very strongly about their roles as school mums. It's like their religion. They're fundamentalist mothers," quips Madeline in the book.
But now that Moriarty's own children (aged 6 and 4) are at school and preschool themselves, the writer had to be careful setting the book in such an close-to-home environment.
"I've got a horrible character - a horrible character - in there with four children, and so I've been saying to any mother I meet with four children, 'You're not the mother!", which really I probably shouldn't, because then they start to look a bit wary.
"I'm very careful not to use real people," she says laughing.
It takes Moriarty about a year to write each book and her ideas can come from anywhere. With Big Little Lies, she was inspired by a writer friend who was attending a school trivia night with a table of friends who were all dressing up as Audrey Hepburn.
"I kept imagining what they'd all look like and then I thought, imagine if everyone dressed like Audrey and imagine if all the men dressed as Elvis." This image became one of the major set-pieces for the book.
The other spark of inspiration came from a friend's child's first day of school.
"Two little girls came out of the classroom and they both had bite marks on their forearms. "Everybody was asking, 'Oh no, what happened?' and they said a little boy had bitten them. Because it was the first day of school no one knew anybody's names and so they got the little boys to line up so the little girls could point out who'd bit them. In the end, the little girls finally admitted that they'd bitten themselves.
"I just loved hearing the story and imagining how you'd feel being one of the mothers and thinking, 'I hope it wasn't my child who did the biting'."
This, too, makes its way into the book and is one of the catalysts of the catastrophes that follow.
Deal-making with Witherspoon and Kidman aside, Moriarty has another novel to deliver by the end of next year.
"I've been joking a lot about how I've had this idea to set [the book] on a tropical island and that I have to do lots of meticulous research to get it right.
"But the more I joke, the more I start to think it could be quite a good setting."
Big Little Lies (Macmillan $37.99) is out now.
