In September, the board decided the book should be sold with a warning that it was suitable for readers aged 16 years and older.
The Office of Film and Literature Classification ruled people might be offended but the book would remain "unrestricted", though it recommended the book be read by a mature audience.
This week, it emerged that the chairman of the board of review, Dr Don Mathieson held serious reservations and would have preferred the book to have had an R18 rating.
Mathieson asked that his "dissenting opinion" be posted on the Department of Internal Affairs website with the board decision.
One of Mathieson's concerns is that the book portrays sex with a 13- or 14-year-old as normal.
"It is injurious to the public good to normalise, as the book does, sexual intercourse by young teenagers," he said.
"Into the River also portrays girls as all too ready for sexual activity. Drug-taking is also presented as the kind of thing that a modern young teenager does; if you don't do it, it is likely you will be disapproved by your schoolmates.
"The book is full of language that is highly offensive to the public in general."
Mathieson said the book had little merit in relation to social matters, and if not restricted, was likely to cause "serious harm to at least some persons under the age of 18".
At the time of the board of review decision, author Ted Dawe said the board had "come down squarely in support of the book's literary values".
Family First national director Bob McCoskrie, who lodged the complaint about the book, said it was disappointing a restricted work was an award-winning children's book.