The Regulator's principal subsidence inspector Dr Gang Li had said old mine workings could be only "marginally stable", and without knowing more, decisions about mining underneath them would be being made "in the dark".

Dr Li's comments to the Independent Planning Commission, made at an October 13 hearing into Wollongong Coal's proposed expansion of its colliery, suggested the miner's claim it would produce "negligible subsidence" may need more scrutiny.

But then Regulator's executive director Anthony Keon wrote to the IPC saying Dr Li's advice was "not the position of the Resources Regulator".

Read more: Post-approval risk assessment an 'outrage', environmentalists say

The Mercury asked the Resources Regulator whether Dr Li's statements were correct.

A response from a spokesman did not answer this, instead confirming the miner could be asked to properly deal with subsidence risks "post-approval".

"The position put forward by Mr Keon was drafted in consultation with Dr Li and are consistent with the views he expressed during the meeting," a spokesman for the Regulator said.

"It is incorrect to state Dr Li was 'overruled'.

"Mr Keon's written response expressly details the concerns raised by Dr Li and confirms the view that further inquiries and investigations must be undertaken by the proponent in relation to the areas specific to the marginally stable pillars."

Nic Clyde, spokesman for anti-coal group Lock the Gate Alliance, questioned whether a mine should be approved without fully assessing subsidence first.

He said a 12-week deadline for the IPC to determine the application imposed by Planning Minister Rob Stokes meant it could not require more information before deciding.

"Ordinarily, we would have confidence that this expert testimony would be enough for Commissioners to send the Russell Vale expansion back to the drawing board," Mr Clyde said.

"But not any more, thanks to Mr Stokes."

The Regulator's spokesman said subsidence could be managed post-approval.

"Similar to other principal mining hazards, these issues can be managed under the specialist WHS framework that NSW has for mines and petroleum sites," he said.

"This will include the development and implementation of a subsidence Principal Hazard Management Plan which requires specific controls [to do with any] old pillar workings that may interact with any proposed or existing mine workings."

We depend on subscription revenue to support our journalism. If you are able, please subscribe here. If you are already a subscriber, thank you for your support.