NSW overvalues library collection by $1b

The body governing NSW's state libraries mistakenly overstated the value of its massive collection of pictures by more than $1 billion.

The collection is one of the most significant in Australia, made up of more than 250,000 artworks and about two million photographs.

The state's auditor-general on Thursday said the Library Council of NSW revalued its pictorial collection in 2017/18 and identified three errors during in its 2015 valuation.

Some assets were double-counted - accounting for nearly $400 million of the overvaluation - while inconsistencies in the way sample data was extrapolated blew the numbers out by almost $600 million.

"All three errors were corrected in the library's 2017/18 financial statements," the auditor-general's report said.

The entire pictorial collection is now valued at $618.5 million.