Vinmonopolet sold 5,428,000 litres of wine, spirits, and beer in September 2013, a 2.5 percent decline on the same month in 2013.
Sales of spirits such as vodka and whisky saw the sharpest drop, falling seven percent on last year to 788,000 litres.
"The reduction in spirit sales can also be understood in the context of record levels of cross-border shopping and in the number of passengers abroad," the organisation said on its website.
Kai Henriksen, Vinmonopolet's Managing Director, called on Norway's new government to act to stem the decline.
"Vinmonopolet's role as an instrument of alcohol policy is under threat," he told Aftenposten. "This is a situation that can not continue and that the new government should now look seriously at."