An offshore oil and gas well in Australia leaked oil continuously into the ocean for two months in 2016, releasing an estimated 10,500 litres. But the spill was never made public by the regulator and details about the well, its whereabouts and operator remain secret.

In its annual offshore performance report released this week, the National Offshore Petroleum Safety and Environmental Management Authority included a mention of a 10,500-litre spill in April 2016. It provided limited details about, noting that it had been identified during a routine inspection.

After inquiries from the Guardian, Nopsema said the leak went on for two months, at a rate of about 175 litres a day. It went unnoticed while the floating platform was undergoing maintenance and was only discovered when the platform returned.

A spokesman for Nopsema said the leak had been caused by a seal degrading. The regulator investigated the spill and said the operator had been ordered to check the seals were working before disconnecting the platform.

But despite requests to reveal exactly where the spill occurred, or what company was responsible, Nopsema refused to disclose the information, revealing only that it was in the North West Shelf.

The Nopsema spokesman said that since companies were compelled by law to report these leaks the regulator believed there was an “implied duty of confidence”.

Andrew Hopkins, an expert in offshore oil safety, said the secrecy was concerning.

“They should release the names of companies concerned because one of the values of this is the naming and shaming approach – that companies that know they will be named in the case of an incident like this, they are going to be less likely to do it.”

When companies get a licence to drill for oil and gas, they produce a safety case that is supposed to demonstrate how they will minimise risks of spills. Those are also kept secret by Nopsema.

Hopkins said if the requirement for checking that the seals worked before they were relied on wasn’t already in the safety case, then that was a failure on behalf of the regulator in ensuring risks were kept as low as possible. “It’s not as if this is an unpredictable unforeseeable event,” he said.

Nathaniel Pelle, a campaigner at Greenpeace Australia Pacific, said: “Australians, and especially those who rely on the ocean for their livelihood, should be deeply concerned by reports that the national oil regulator has withheld information from the public about a 10,500-litre oil leak for over 12 months.

“There’s absolutely no justification for continuing to keep the company involved or the location of the oil spill a secret.

“Any spilled oil is unacceptable but the US oil industry suffered three spills over 8,000 litres in 2016, despite producing 30 times the oil that Australia does. It’s farcical that the industry claims to have a safe record in Australia.”

In the report, Nopsema noted that there were no fatalities in the industry in 2016 and that injuries and accidents were down. But the report found that spills were up 28% over the previous year, despite activity in the industry dropping 40%.

“Nopsema’s performance report should be a wake-up call to the government and to anyone who has the bad luck of sharing the marine environment with the oil industry,” Pelle said.

Nopsema noted that the number of leaks isn’t an environmental indicator but a safety one, since some leaks could be small and intentionally done for safety reasons.

“Overall safety is improving with all other incident indicators trending down, with 2016 being the first year since the establishment of a national safety regulator in 2005 (NOPSA), that there has been no fatalities or major injuries,” the Nopsema spokesman said.