Kiwi children's book The Wonky Donkey is a viral sensation after video of a grandmother reading the tale to her grandson gathered millions of views.

Scottish granny Janice Clark was caught on film reading The Wonky Donkey to her four-month-old grandson Archer and she couldn't contain her laughter.

Since the video went viral days ago, book lovers all around the globe have been searching for the book, by New Zealander Craig Smith.

The Wonky Donkey is based on Smith's song of the same name and tells the tale of a three-legged donkey.

The book about the wonky donkey adds a new adjective every few pages, ending with "spunky, hanky-panky cranky stinky dinky lanky honky-tonky winky wonky donkey".

The Scottish granny finds the adjectives hilarious and giggles her way through the book.

"Oh dear, how can anybody read this seriously," she says at one point. "This is going to kill me."

The video has been viewed more than three million times in the last week alone.

The Kiwi author of the book told the Guardian that his publisher is "rushing to print another 50,000 copies, with a view for more" as demand for the book has soared since the video hit the internet.

The book, illustrated by Katz Cowley, had already sold more than one million copies before the online video turned it into a sold-out bestseller.

Do I know anybody in NZ that can buy a book and ship it to me? Trying to get a copy of Wonky Donkey by Craig Smith but it’s ~$400 in the US and like $14 in NZ.

— Josh Meier (@moshjeier) September 8, 2018

"The video is gold. Watching Janice read and laugh was just delightful, and like many, her infectious laugh had me laughing too," Smith told the Guardian.

"I've always wondered why sales had not taken off so much in the UK and US, but that looks like that's about to change."

"I am so glad that Wonky's reputation has finally come to bite him on the bum on this side of the world thanks to 'Wonky's Guardian Granny'. I couldn't be more delighted," Smith added.

Out of curiosity, I wanted to see if Amazon had any copies of The Wonky Donkey for sale... pic.twitter.com/3wp9nN8XGC

— Justin Bradford (@justinbbradford) September 10, 2018