Water had spread across five levels of the building, including the basement, before firefighters could shut the sprinkler off.
Fire Service northern communications shift commander Scott Osmond said the water was putting a collection of heritage books, including some of international significance, at risk.
He said six fire crews were still at the building working with library staff to prevent further flood damage.
The sprinkler could have malfunctioned or been mistakenly set off, he said.
An Auckland Council spokeswoman said most of the water had been cleared by 2:45pm.
Many of the books may have been saved by plastic sheets set up on the top of shelves, she said.
Closed till Monday, Academy Cinema closed Saturday hope to open Sunday.
Thanks to Bookbrainz for this info.