IT was meant to be a remediation program to repair extensive mine subsidence damage to Sugarloaf State Conservation Area.

Instead it turned one environmental disaster into another.

Contractors working for coal giant Glencore Xstrata pumped more than 180 tonnes of concrete into a tributary of Cockle Creek at Lake Macquarie.

The concrete creek, uncovered as part of the Newcastle Herald's investigation into mine subsidence at the state conservation area, stretches more than 400 metres down a hillside and dries up less than a kilometre from Cockle Creek that leads to Lake Macquarie.

In some places it is more than one metre deep and five metres wide.

A worker who had been involved in the remediation project described the creek grouting as an "absolute catastrophe".

He said Orica Mining Services employees, contracted by Glencore Xstrata, were attempting to grout a large subsidence crack at the top of a ridge in the reserve in June.

They were unaware that more than 12 concrete trucks worth of grout was spewing out another crack at the bottom and running straight into the creek.

To read the Herald's opinion, click here.

"There was an unsupported ridge above the grouting area, so there was no one monitoring the end of the pipe," he said.

"It was running for a very long time before anyone realised. I have no idea how it can be cleaned up, the problem is just too massive."

Discovery of the creek comes just a day after Glencore Xstrata's spokesman assured Hunter residents that the mine was working to "provide appropriate remediation of mining impacts and operate in a responsible manner".

Last night he said the mine was "extremely disappointed" by the subsidence damage at the reserve and admitted it was "greater than predicted".

He said West Wallsend Colliery was fully committed to "working with all parties" to remediate the creek.

Almost three months after the creek was grouted and on the day the Herald revealed the mine subsidence damage in the reserve, the Office of Environment and Heritage moved last night to force the company to clean up the mess.

Chief executive Sally Barnes, who described the area's conservation value as "very great", said an agreement was made to cease the mine's remediation project after the creek was grouted in June.

"We have given them a direction today to remove the grout . . . it's a legal direction and they have until September to come up with a plan," she said.

"We have had specialist investigators up there and we are looking at regulatory action that might be appropriate, including possible prosecution."

Environment Minister Robyn Parker said yesterday she was "concerned" about the "two reported incidents in relation to mining activity below Sugarloaf State Conservation Area".

"I'm advised that the National Parks and Wildlife Service has requested a review of the current subsidence plan, including advice of potential future impacts from the Department of Planning and Infrastructure," she said.

In May, Ms Parker responded to complaints raised by adjoining landholder Robert Bignell about areas of the park being cordoned off, informing him the mine was operating with the "appropriate level of environmental assessment and monitoring".

The Herald revealed the wide-scale subsidence damage to the conservation area, caused by subsidence from West Wallsend Colliery, yesterday.

Cliffs have crumbled or collapsed, a huge chasm has opened up and kilometres of cracks stretch across the reserve.

The subsidence was tracked over more than two kilometres within the ecologically sensitive conservation area adjacent to the underground mine's longwall 41.

A chasm 120 metres long and 17 metres wide opened up following a large subsidence event on October 2 last year.

In 2010, plans on display with the state government showed the workings above the reserve were expected to create up to 2.5 metres of subsidence.

The exact extent of the damage to the reserve is impossible to determine due to the steep terrain.

Grouting has been going on in the area since June last year and thousands of tonnes of the aerated cement has been pumped into cracks and holes.

The mine has two full-time security guards stationed in the park in an effort to monitor access to the popular bushwalking and mountain bike riding reserve.

Crack repairs have been done on Archery Road, the dirt road that enters the reserve from Wakefield.

The worker described some of the cracks in the reserve as "enormous" and said they could "easily swallow a motorbike and rider".

"They would just disappear and no one would know, it's that bad in some areas," he said.

"There would be about nine or so affected sites, it's very widespread."

The area is cordoned off with security tape labelled "no road" and has signs indicating the area has been affected by mine subsidence.

The mine was granted approval by the Department of Planning and Infrastructure in January last year to extend its underground operations under the conservation area.

Approval included the longwall 41 that caused the subsidence.

A spokeswoman for the Division of Resources and Energy said subsidence predictions from underground mining were based on "expert opinion of the likelihood of surface impacts".

"It is not an exact science and can depend upon a large number of factors including depth of the workings," she said.

In 2011 state environment department officials said the mine could damage the conservation area and urged Glencore Xstrata to avoid mining at low depth in the reserve.