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An award-winning children's author has defended his book against charges of plagiarism, saying there is no copyright on a joke.

Craig Smith won the Children's Choice Award at the NZ Post Children's Book Awards less than two weeks ago for his book The Wonky Donkey.

Mr Smith says he adapted the book from a children's song he wrote in 2005, based on a joke he heard from friends.

The book turns the old joke - what do you call a donkey with three legs? A wonky donkey - into a tale of how a donkey becomes a "spunky, hanky, panky, cranky, stinky, dinky, lanky, honky, tonky, winky, wonky, donkey."

An executive at Mr Smith's publisher, Scholastic New Zealand, says jokes cannot be plagiarised because they are in the public domain and finding the copyright holder is impossible.

But intermediate school teacher Margaret Marsick says she has found versions of the joke online and the basis for the book should have been referenced in the book.

Mr Smith and Scholastic say the jokes found on the internet were placed there after the song was recorded and sold on CD.

Mr Smith maintains the song stemmed from a short version of the internet jokes and that he did not see the longer version until being told about it years later.

"I've never ever made a secret of the fact that the inspiration to the song was from a joke ...

"I have never said this was 100 per cent my idea - never ever."

Asked if those who posted the joke online had plagiarised his song, Mr Smith said: "I don't think so, I think the joke's been around for a long time."

The accusation had put a dampener on his award.

"It is a little bit of a hard thing to have to deal with ... I think she's picking on my one maybe because everyone likes it."

Scholastic publishing manager Diana Murray said it was a shame someone was dampening down the book's success.

"There are overlaps with this longer joke that are available online in the public domain but I think that he came up with it independently.

"It's just done so incredibly well. We've sold nearly 40,000 copies of it in New Zealand which is just huge and it's been in the overall top 10 best sellers.

"And that's alongside Jodi Picoult and Bryce Courtenay and writers of that ilk - for months.

"We haven't seen a success like this for a very long time."