In the night between March 18 and 19, the company became the target of a "massive" cyber attack involving ransom ware, forcing it to disconnect from various sites and factories and switch to manual operations in others.
The attack also forced it to postpone the publication of its quarterly earnings, originally scheduled for Tuesday, to June 5.
Norsk Hydro estimated that the overall financial impact of the cyber attack would be 400-450 million Norwegian krona (USD 46-USD 52 million, 41-46 million euros, but also said it had "robust" cyber insurance.
The light metals producer said it had not paid any ransom and has filed a complaint with Norwegian police, which has opened an investigation.
The company's shares dropped 1.65 per cent in morning trading on the Oslo Stock Exchange.
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