With more than 50 books published and many millions sold, Pamela Allen is one of Australia and New Zealand’s most remarkable and memorable children’s authors. Her picture books – including classics like Who Sank the Boat?, Mr McGee, Alexander’s Outing, and Bertie and the Bear – established her as a master of playful and inquisitive children’s literature, taking young readers on warm, gentle misadventures.

At 90 years old, she has written a new book: Mr McGee and His Hat, which sees the return of the beloved stocky man, his little bowler hat and unplanned escapades. It has been seven years since she last wrote a book, and 12 years since Mr McGee’s previous appearance.

“It’s a little idea for a little child. But I think it’s fun if you’re two or three,” Allen says. “Mr McGee is imaginative. It’s nonsense. And it’s intended for the very young, who will be delighted in my little bit of nonsense.”

We talk on her last afternoon in Melbourne, visiting her family to celebrate her 90th birthday. While her son packs their luggage and her grandchildren play loudly in the other room, we sip lemongrass tea in the kitchen. It’s rare to find a moment’s peace in her daughter Ruth’s glass-blowing studio, with students (including me) walking in and out, wondrous creations in every corner. But it’s part of the charm of the artistic life.

Since her husband, the prominent New Zealand sculptor Jim Allen, died last year aged 100, Allen has been rediscovering herself both as an artist and as a person. “I had to re-establish my mental health among the living,” she says. “And the way in which I could do that was to write a book. And I consciously put myself as a first priority, after his death, to re-establish my sense of worth. Because you lose all contact with the living drive that exists. But if death is forever a prison, you’ve got to climb out of it. So that’s why Mr McGee was a natural resource for me.”

Allen was adamant that Mr McGee’s clumsy fate not be revealed until opening the pages of the book, even arguing with her publisher over the original title, which she complained gave too much away. She is passionate about allowing her readers to make up their own minds about her books. For instance, it wasn’t necessarily the mouse who sank the boat in Who Sank the Boat?, she says: “I’m inviting the child to tell me.”

It wasn’t just writing a new book that has brought Allen back to herself after her husband’s death. She has also taken up painting, having drawn her illustrations since graduating art school in the 1950s. “Jim was painting. He was an artist. In the process of dying, he was painting. So when he died, all his stuff came into my little apartment, many colours and brushes. So I’ve started painting and I’m enjoying it. In fact, I’m getting quite fixated on it … I’ve been telling everyone I’m very good at it.”

You won’t be seeing any Pamela Allen originals on the art market, however, and she won’t say much about what it is she is painting, not even allowing any photos in her studio. “Its purpose is to give me strength and pleasure in being alive,” she says.

Allen’s publishing career spans over 44 years. She credits her beginnings to the “Playcentre” movement in New Zealand, where communities of parents began to cooperatively educate children in the 1940s. With her two children in a Playcentre in the 1970s, Allen would read books to small children and soon learned what they liked and didn’t like. In both Australia and New Zealand, the 1970s was the beginning of a local children’s book industry, departing from imported British texts. Allen’s career took off after she illustrated two books by her friend Jan Farr, and her own debut, the acclaimed Mr Archimedes’ Bath, was published in 1980 after meeting Anne Ingram at Collins in Sydney, who encouraged Allen to consider writing, not just illustrating.

“I knew I could draw … I walked home across the Harbour Bridge and I thought, ‘Right, Pam, you’re an author’,” Allen says. “I’d never given it any thought about being an author.”

Central to all of Allen’s books, which “sit in relationship with different ages”, is the drama. Steam trains, sinking boats, cats who won’t stay home and monsters who will; rhyme and pentameter abound.

“I write my books for children who cannot read,” she explains. The first connection very small children have with books is the voice of their parents: “The voice can do all kinds of things. It can sing. It can admonish, it can create music. It can have a rhythm. And the parent is important, because they create the drama with the voice.”

“After the voice, the child will start engaging with the pictures. And you’ve got the sharing process. So there’s a dialogue between the adult and the child, which creates a togetherness.”

Allen starts with the pictures, and never writes what can be drawn, but she agonises over every word: “I’m anxious and busy trying to select the exact word, because the child doesn’t know the meaning of it yet.”

In January, she was surprised to be awarded a member of the Order of Australia for her services to literature, almost a decade after she received the equivalent honour in New Zealand. The Australian honour was a “very nice” reminder of how loved she is here.

“You are who you are. And in one sense, it hasn’t made a lot of difference to me, because I’ve always known who I am and what I’m doing and knew it was good,” she says. “I wholeheartedly believed in what I was doing. I had my own concept of what to do in a children’s book. And I wasn’t terribly interested in other people’s children’s books, because I knew what I was on about.”

Allen is now 90. Will Mr McGee and His Hat be her last book, or will she write more?

“If I have a good idea!” she laughs gently.

• Mr McGee and His Hat by Pamela Allen is out on 2 July