"Having it for your own personal use is okay. Passing it around to your friends is not," she said.
Mr Dawe said he was "blindsided" by the ban, which was sought by lobby group Family First after deputy chief censor Nic McCully removed a previous R14 restriction on the book on August 14, making it totally unrestricted.
"It's extraordinary," Mr Dawe said. "I've had quite a few emails from people who share that sense of outrage. Do we live in a country where books get banned? I'll get burnt next."
He said Family First director Bob McCoskrie and Dr Mathieson, who wrote a dissenting view advocating an R18 restriction when the majority of the board rated the book R14 in 2013, were overstepping the rules of a democratic society.
"Those two individuals are united in their determination to establish this as a line that will not be crossed. I feel they have wildly overstepped the whole mechanism of looking at art and making judgments on it," he said.
"New Zealand has taken a giant step towards that sort of regulatory moralising that I think most people felt we had left far in our past."
He said it was not easy to write a book that teenagers would want to read, or to get it published.
"People involved with teaching boys, especially English teachers, know how important books like this are because they speak to boys about the things that other boys' books don't have the firepower or the vitality to do effectively," he said.
"The book was never about sex and drugs, it was always about bullying people and how that damages people for the rest of their lives. That is really the underlying theme, everything else is just the trappings that go along with that."
Ms McCully's decision last month quoted another writer for teenagers Bernard Beckett as saying that sex, violence and bad language were common in books that were taught in schools such as his own 2014 novel Home Boys, which "includes a boy showing his friend how he masturbates, and ends with an explicit sex scene".
"Thinking back to the classic school texts, Catcher in the Rye started it all," he said. "A Clockwork Orange is as brutal as they come, and is frequently taught in senior school."
Libraries Association director Joanna Matthew said Auckland Libraries submitted a British graphic novel Lost Girls to the censor this year because it included images of sexual activity by children. The censor rated it R18.
But she said libraries generally supported freedom of speech and saw the ban on Into The River as "a tragedy".
"If we censor literature that talks frankly about some of these issues, then I think we run the risk of burying them," she said.
"We would be much more effective as a society if we worked to counter the problems that the book articulates rather than trying to restrict the book."
The NZ Booksellers Association has placed a notice on its website warning bookshops that they face fines of up to $3000 for an individual or $10,000 for a business if they supply the book.
However the book is still on sale on Amazon at US$24.99 in paperback or US$9.99 on Kindle.
