The owners of a longwall mining operation have been ordered to fix a damaged creek in NSW.

Redbank Creek in south-west Sydney has been cracked, shattered, and left in disarray due to mining subsidence — a result of underground longwall mining.

At GFG Alliance's Tahmoor Mine, longwall mining is used to extract coal from underneath Redbank Creek. 

The NSW Government has just a few months to come up with a way to fix the problem.

“Environmental impacts have exceeded performance measures identified in the mine's approved Environmental Management Plan, which has triggered the requirement for a remediation plan,” the NSW Government has told reporters.

“The Resources Regulator has directed that the titleholder develop that plan no later than 31 December 2018,” it said.

One of Australia's leading water scientists, Dr Ian Wright, has been studying the effects of mining subsidence on Redbank Creek for about five years.

In a new report, various points along the river were tested before, during, and after sections of bedrock were damaged.

“The scale of the pollution that we've seen, as far as we know in science publications, this is the worst scale that we've ever found in the literature,” he said.

Water in downstream sections of the creek were found to be high in salt and toxic metals that had slipped underground and mixed with underground aquafers.

Geochemist Dr Jason Reynolds says; “We have a situation where we have multiple metals reaching the surface, combined with dissolved oxygen levels”.

“[It] really gives no chance for the ecosystem to survive.”