The surplus stood at 6.8 billion kroner ($825 million, €729 million) in August, a drop of 67.5 percent compared with a year earlier, the national statistics institute SSB said.
Exports fell by 13.1 percent, hit by natural gas sales that plunged 42.2 percent, with imports increasing by a nearly 11 percent.
Outside of hydrocarbons and shipbuilding, Norway's trade balance had a deficit of 19.4 billion kroner.
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Since the beginning of the year, Norway's trade deficit has shrunk by 48.3 percent compared with the same period in 2015.