Hanging on to an overdue library book has long been seen as a victimless crime and now, in Sydney at least, it has no punishment.

The City of Sydney Library has given fines for overdue books the flick, after a successful trial that saw 67,945 volumes returned to their shelves — about three times better than previous levels.

Some of the books returned during the seven-month trial had been missing for more than 10 years.

Sydney Lord Mayor Clover Moore said it was a case of reverse psychology.

"Library fines have always been used to deter people from leaving their books at home," she said.

"But we've found that in most cases they had the opposite effect, frightening members into never returning their overdue items.

"This new approach encourages positive community responsibility and sharing, rather than penalising people."

The City of Sydney Library has decided to waive fines at its nine branches until 2021.

Sydney Lord Mayor Clover Moore said the trial was a success. ( AAP: Marianna Massey )

'It's never too late'

Lynette Makin, CEO of the New South Wales Public Library's Association, said fines amnesties had a history of success, citing similar results from her time in charge of the Upper Murray Regional Library.

"We actually appealed to people and said if they didn't return the books, that other people couldn't borrow them," she said.

"That worked beautifully. We went from having a third of our collection overdue, to under 10 per cent."

"After that, our overdue items stayed a very low level."

Ms Makin urged people to return their overdue books, even if they were embarrassed.

"It's never too late to hand a book back, sometimes when you hand them back we don't know, it might be very valuable," she said.

"Around the world you hear stories about books that get returned after 50, 60 or 70 years."