Freedom of expression needed to be weighed against the need to protect the public good and young people, he said.
"Where a book is targeted at teenagers it needed to be language and theme appropriate."
Mr Dawe could not be contacted yesterday.
The order is the latest twist in an extraordinary saga for Into the River, which won the top prize in the 2013 Children's Book Awards. The censor's office first classified it as unrestricted with a note about explicit sex, drugs and offensive language. The review board later imposed an R14 restriction, but this was overturned last month when deputy chief censor Nic McCully ruled that the book should be unrestricted.
Dr Mathieson, who argued a minority case for an R18 restriction in 2013, said in the new interim order it was "debatable, and a matter of independent public interest, whether the chief censor acted lawfully" in overturning the board's decision.
"The correct classification of Into the River under the act will operate as a semi-precedent and will exert a significant influence upon other decisions portraying teenage sex and drug-taking," he said.
Publishers Association president Melanie Laville-Moore said the book was a prizewinner in a strongly contested competition, a fantastic piece of writing and if it was deemed inappropriate it would not have won. The ban was disappointing for the author and she hoped good sense would prevail.
Media law expert Professor Ursula Cheer said it was now illegal to supply the book to anyone until the full board made a final decision.
"Having it for your own personal use is okay. Passing it around to your friends is not."
Interim orders had been used for film festival movies but no one had used the process for books.
"You don't get many books going through the system. There's kind of a view that they are less dangerous than online stuff," she said.
Booksellers Association chief executive Lincoln Gould said bookshops would have to remove the title from their shelves if they were aware of the order. Libraries and schools could not supply it.
The saga so far
June 2013: Into the River wins top prize in NZ Post Children's Book Awards.
July: Internal Affairs Department submits it to the censor after complaints from the public.
September: Censor classifies it M (unrestricted) with "contains sex scenes, offensive language and drug use".
December: Review board partially upholds Family First appeal and imposes R14 restriction.
March 2015: Auckland Libraries asks the censor to reconsider the classification.
August 14: Censor reclassifies the book "unrestricted" with no descriptive note.
August 18: Family First appeals to review board again.
September 3: Review board president bans book until full board decision.
