But environmentalists say water monitoring is inadequate after heavy rains washed large amounts of coal fines down Camp Gully Creek and into the Hacking River in the national park.

The Environment Protection Authority (EPA) this week gave Peabody another clean-up notice, the third this year, directing the US-owned miner to clean large amounts of built-up coal fines out of its retention dams so they can properly contain large amounts of water.

Environmentalists have pointed to the miner's water monitoring reports which show that for nine months in 2021, and five from eight months this year, valid water samples were not taken from the discharge point listed in its EPA pollution licence.

The only monitoring location Peabody is required by the EPA to report is called Point 9, the clean water tank at the mine's water treatment plant. This is for water that is to be discharged into Camp Gully Creek.

Read more: Coal pollution extended throughout national park to Audley

Peabody says the months without samples were occasions when no water was discharged into the creek.

But the pollution incidents in February, July and September this year were caused by runoff in heavy rain that did not pass through the treatment plant on its way to the creek.

Dr Ian Wright, senior lecturer at Western Sydney University, said proper monitoring was needed downstream because the EPA licence did not capture the reality of the sensitive area.

"It's not so much what's coming out of that waste pipe, it's the overall impact on Camp Creek and the Hacking," he said.

"The runoff in storms isn't captured by the licence."

He said there was precedent at South32's West Cliff mine (now part of Appin).

"In their licence they've actually instituted monitoring above and below the inflow of [their] waste," he said.

A spokeswoman for Peabody said clean-up efforts moved into the national park last week.

"This is the first time remediation activity has been approved inside the park," she said.

"This approval allowed a team of six people to remove coal sediment from above the waterline at the confluence of the Hacking River and Camp Gully Creek and that activity was completed last week.

"As has been the case for many weeks, most areas of the Creek and adjoining waterways are now substantially clear of in-stream coal sediment.

"The company has provided the results of regular water tests taken since the incident to the Environmental Protection Agency.

"Encouragingly, independent water quality tests taken soon after the event showed that conductivity (a measure of salinity), pH, dissolved oxygen and turbidity levels measured at sites downstream from our site were similar to levels measured at sites upstream of the mine."

An EPA spokeswoman said the Metropolitan Colleries licence was now under review "to ensure that the sampling requirements are robust".

"The EPA has been collecting water samples downstream of the discharge location since mid-2022," she said.

"Clean-up of impacted areas is continuing and clean-up has started on the confluence of Camp Gully Creek and the Hacking River."

It''s understood Metropolitan Collieries discharges water through two licensed discharge points. Routine monitoring is required at the wastewater treatment plant discharge point. Monitoring is not required at the spillway discharge point.

Neither the EPA nor Peabody have conceded how far the pollution extended, but witness reports revealed discolouration at the Audley Weir - meaning the pollution had penetrated almost the entire length of the park.

The EPA's latest clean-up notice says the buildup of coal material in the dams can cause pollution problems.

"The EPA reasonably suspects that a pollution incident has occurred and is likely to occur from the premises with any stormwater discharge," it states.

"That is, the retention of large volumes of fine coal material in the dams, and the associated reduction in storage capacity and retention time, means that any discharge from the dams is likely to result in a pollution incident."