The NSW Government has ordered GFG Alliance's Tahmoor Mine to repair the damage and cracks to a creek in south-west Sydney, where water no longer trickles in some sections.

Key points: Water is "disappearing" through cracks at Redbank Creek in southwest Sydney

Water is "disappearing" through cracks at Redbank Creek in southwest Sydney The damage is a result of underground longwall mining by the Tahmoor mining operation

The damage is a result of underground longwall mining by the Tahmoor mining operation The NSW Government has told the mine's owners to come up with a plan to fix damage to the creek

Redbank Creek, which "runs" near Picton, is the subject of significant environmental concern.

The creek's bedrock is in disarray in some places and the damage is caused by mining "subsidence" — the result of underground longwall mining.

Underground longwall mining is a common form of mining in Australia, and in this case, it is carried out by GFG Alliance's Tahmoor Mine which extracts coal from underneath Redbank Creek.

The NSW Government has ordered the owners of the Tahmoor Mine to come up with a way to fix the problem before the end of the year, the ABC can reveal.

"Environmental impacts have exceeded performance measures identified in the mine's approved Environmental Management Plan, which has triggered the requirement for a remediation plan," a NSW Government spokesman said in a statement.

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

In a statement, GFG Alliance's mining chief operating officer Matt Reed said the company was committed to meeting "the highest of environmental standards".

"Remediation of Redbank Creek is anticipated to take up to five years to complete and is expected to result in progressive improvements over that time," he said.

He said monitoring of stream flow and ground water levels would continue, and that the company would work with the NSW Government to rehabilitate the creek.

Redbank Creek is highly fractured when it should be solid, according to scientist Dr Ian Wright. ( ABC News: Jonathan Hair )

Some water 'high in salt and toxic metals'

One of Australia's leading water scientists, Dr Ian Wright, said the fact there was no water in the creek was "definitely unnatural".

"We're sitting, standing, and walking around on highly fractured sandstone, this should be solid bedrock," he said.

"When there has been heavy rain and we've seen flow down the creek, we have literally seen water flowing down the creek and then disappearing down into some of these cracks."

Dr Wright has been studying the effects of mining subsidence on Redbank Creek for about five years, and has released a new study on how it is affecting water quality.

Points were tested before, in the middle of, 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.

The water in sections of the creek downstream from the subsidence were high in salt and toxic metals after it had slipped underground and mixed with underground aquifers.

Dr Jason Reynolds, an expert in geochemistry, said it is "never one specific metal that has an adverse effect".

"So here we have a situation where we have multiple metals reaching the surface, combined with dissolved oxygen levels," Dr Reynolds said.

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

Resident believe it is too late

Local residents like David Hunt, a former engineer, are disappointed the damage has been allowed to happen to their creek.

Local David Hunt is disappointed that longwall mining has damaged Redbank Creek. ( ABC News: Jonathan Hair )

"Anyone who's actually looked into it can't believe that we've allowed it to happen on our watch," he told the ABC.

"And that's what we're hoping now is that if we can actually show what's happened, and what will always happen when there's longwall mining underneath any area.

"Hopefully then in the future it will be looked at differently and it may not be allowed to occur."