The extent of weak or "marginally stable" pillars from earlier mining was not known, meaning subsidence could not be fully assessed, NSW Resources Regulator principal subsidence inspector Dr Gang Li told the Independent Planning Commission.

"Without a reasonable understanding of this key risk factor, we are in the dark in making decisions in relation to Russell Vale colliery's proposed revised underground expansion project," Dr Li told the IPC on October 13.

This concern was later "clarified" by the Regulator's executive director Anthony Keon, who said in a letter to the IPC that these concerns could be worked out after the mine was approved.

Read more: Council claims 'no public comment' on mine expansion

"[The] Resources Regulator's position remains that the identified risks can be suitably and appropriately managed post approval provided that appropriate inquiries and investigations are undertaken by the proponent to further identify and define the existence and distribution of the marginally stable pillars ..." Mr Keon wrote.

The marginal pillars concern relates to the Bulli Seam of coal, which sits above the seam sought to be mined under the current expansion plan. If the pillars are weak, mining beneath could have more significant consequences.

Gavin Workman, from the group Illawarra Residents for Responsible Mining, said it was an "outrage" to suggest approving the mine then working out subsidence concerns afterwards.

"Wollongong Coal still hasn't met its obligations for its previous approvals at Russell Vale," he said.

"It is an outrage that NSW Planning's solution is to approve this expansion and then get Wollongong Coal to inspect all the 'marginally stable pillars' in the Bulli seam to see whether they are strong enough to hold the whole place up.

"It is totally inappropriate that we should risk our drinking water by letting a mining company self-assess the likelihood of major damage after it has been given approval to mine."

Mr Keon also recommended mining start from west of Mount Ousley Rd, to "allow time to investigate and clearly identify the marginally stable pillars in areas east of Mount Ousley Rd".

The Mercury sought comment from the NSW Department of Planning, Industry and Environment, which includes both planning authorities and the Resources Regulator.

A statement in response was said to be attributable only to "a spokesperson for the NSW Government".

"The comments made in the meeting did not form part of the Resources Regulator's advice to the Department during the assessment process, and the Regulator has since confirmed in writing that these issues can be suitably and appropriately managed in accordance with conditions of consent, if the project is approved," it said.

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