Council agents fix the valuation of international works at what they would fetch on the London market - in British pounds sterling. But the council's financial statements are prepared in New Zealand dollars.
Regional Facilities Auckland spokesman Paul Brewer said the gallery used London fine art auctioneer Christie's to value its international works this financial year.
"Given the strength of the NZ dollar over the last couple of years, the values have decreased in NZ dollars but remained the same in pounds sterling.
"Putting this into context, the total art collection - international and New Zealand works - was valued at $261 million and so a $4 million reduction represents only 1.5 per cent."
Mr Brewer said the valuation was for insurance and financial reporting purposes.
"For insurance and security reasons we do not reveal the valuations of individual works of art."
Gallery head of collections Catherine Lomas said a further influence on valuations was that values could change according to the market.
"The art market is a changing one and artworks rise and fall in popularity. So, valuations are based on recent art market sales, reflecting the popularity of works with art buyers."
In addition to professional advice from London for valuations, the gallery also uses Webb's and John Gow to value expensive New Zealand works.
Over the years, the gallery's collection had increased in value mainly because of its collection of 19th century Victorian painting and its print collection, said T.J. McNamara, who has written about the arts for the Herald for 48 years.
The Victorian painters included Lord Frederic Leighton and Sir John Everett Millais whose work faded in popularity for a while but was now "worth millions".
McNamara said the Auckland gallery had built a collection as good as those of provincial galleries he had seen in Bristol and Aberdeen and in Australia.
"It's been unrecognised just how good Auckland City Gallery's collection is."
The gallery owns 73 per cent of its collection and adds to it through purchases, gifts, bequests and collections on long-term loan. It has more than 100 McCahon paintings and 70 works by Gottfried Lindauer.
Art numbers
$261m
value of Auckland Art Gallery's collection in 2013-14 annual report
$21m
rise in past five years
$4m
drop expected after fresh valuation for insurance cover