THE planned expansion of a coalmine near Ryhope was likely to create "a high level of damage", the NSW Office of Environment and Heritage says.
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Glencore Xstrata's West Wallsend mine was found last year to have caused damage to Sugarloaf State Conservation Area.
The subsequent remediation work was botched when tonnes of grout used to repair subsidence cracks flowed into a creek, which the mine later cleaned up.
The NSW Planning Department is assessing Glencore's expansion plan in Sugarloaf, which includes two new longwall panels.
Subsidence of 2.4 metres to 2.6 metres was predicted in some areas.
The mine said it was committed to fixing any subsidence, so public safety was not affected.