OSLO, April 15 (Reuters) - The discovery of a faulty valve on the North Sea gas transportation hub Sleipner Riser cut supplies from Norwegian onshore processing plant Nyhamna, gas system operator Gassco said on Monday.

The outage commenced on Sunday when a valve did not tighten as planned, Alfred Hansen, director for system operation at Gassco, told Reuters.

Sleipner is a key transport and gas-hub for gas from Kollsnes and from Nyhamna for further transport to Draupner, Zeebrugge and Easington.

While the incident had also impacted other operations, it was predominantly Nyhamna that remains affected, he added.

"We just had a meeting with the field operator and they have a plan to improve the situation during the night," Hansen said.

Operator Equinor was not immediately available for comment.

Gassco will update the status for Nyhamna as and when it receives new information, Hansen said.

The latest information posted by Gassco on its website shows that Nyhamna's technical capacity of 79.8 million cubic metres (mcm) per day will be curtailed to 10 mcm/day until 0400 GMT on Tuesday. (Reporting by Nora Buli, editing by Terje Solsvik)