The company said on Wednesday that its technical team, with external support, had found the root cause of the problems and was working on a plan to restart the company's IT systems "in a safe and sound manner".

The attack was said to have started late on Monday. Norway is 10 hours behind AEDT.

Note that despite being extremely open about the scale of the issues with public and media and putting execs in front of streams talking about an “extreme” situation, Norsk Hydro’s share price is fine. Compare that to where companies have hidden and minimised things. pic.twitter.com/8WzrOq2Xsd

— ? Kevin Beaumont ? (@GossiTheDog) March 20, 2019

The firm said it had isolated all plants and operations and switched to manual operations and procedures, with the main priority being to ensure safe operations and to limit the operational and financial impact of the Windows ransomware.

"I'm pleased to see that we are making progress, and I'm impressed to see how colleagues worldwide are working around the clock with dedication to resolve this demanding situation and ensure safe and sound operations," says chief financial officer Eivind Kallevik.

"I would also like to complement our external technical partners who have done an important job in supporting our efforts, and also relevant authorities, who handle the issue with the diligence it deserves."

Norsk Hydro said most operations were now running and ensuring deliveries to customers according to specifications, "with some more manual operations than normal".