A leak from an offshore oil well in WA has been publicly reported more than a year after it happened.

The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) referenced the 10,500-litre spill in its annual offshore performance report last week, even though the safety authority was notified in April 2016.

NOPSEMA did not name the operator or location, but media investigations have revealed the leak occurred at a Woodside Petroleum well in the Cossack field on the North West Shelf.

In a statement to reporters, Woodside said the leak continued for 60 days at a maximum rate of about 175 litres per day, resulting from a “seal degrading on a subsea hydraulic valve control line”.

The company says it caused no environmental damage.

“Due to the composition of the fluid, small quantity released, water depth at release site and distance from environmentally sensitive areas, there was no lasting impact to the environment,” the statement said.

“We remain a committed, responsible and prudent operator and continue to adhere to transparent reporting and high standards of operations that have characterised our history.”