Norsk Hydro said the March cyber attack that paralysed its computer networks would cost the aluminium maker up to 450 million Norwegian crowns ($52 million) in the first quarter.
The Oslo-based firm, one of the world's largest producers of the light-weight metal, was forced to halt some production on March 19 and switch other units to manual operation after hackers blocked its systems.
Hydro has since postponed its first-quarter earnings report by five weeks to June 5 from April 30, amid efforts to restore systems for reporting, billing and invoicing.
The company reiterated that it has a cyber insurance policy to help cover the cost of the attack.
The Norwegian National Security Authority, the state agency in charge of cybersecurity, said the attack used a virus known as LockerGoga, a relatively new strain of so-called ransomware, which encrypts computer files and demands payment to unlock them.
Hydro has maintained it will not pay any ransom to regain access to its computers and servers, and instead preferred to repair data from backup systems. The company's Extruded Solutions unit, which turns slabs of aluminium into finished products for car manufacturers, construction companies and others, was affected the most by the cyber attack.
"Preliminary data shows that external sales volumes in Extruded Solutions fell to 333,000 tonnes in the first quarter from 362,000 tonnes in the same quarter last year," Hydro said in a statement.
The unit had planned for volumes to be "somewhat below" the year-ago levels, but the cyber attack further reduced its output, it added.
While other units, including Bauxite & Alumina, Primary Metal, Energy and Rolled Products had been able to produce at close to normal, they still required a higher degree of manual operation, it added.
($1 = 8.6563 Norwegian crowns)
Source: Terje Solsvik and Izabela Niemiec for Thomson Reuters, Apr 30