The NSW Resources Regulator has confirmed that it had ordered the reassessment, performed by an independent expert, and was now investigating the discrepancy.

The State Government currently holds $7.5 million for environmental rehabilitation in case the company goes under.

But as for the biggest discrepancy - between the miner's and the Regulator's rehabilitation figures - this has not been explained.

Wollongong Coal has more than $1 billion in liabilities and warned recently that it would cost $215 million to rehab the site - money it doesn't have - if it didn't get approval for its underground expansion.

A spokesman for the Regulator said it believed the new figure of $12.4 million was sufficient.

"The NSW Resources Regulator is satisfied that the revised security is adequate to cover the full cost of required rehabilitation activities," he said.

"The Regulator is currently investigating the discrepancy between the assessed security amount and the information that the company provided as part of the submission of the EIS documentation."

Nic Clyde from anti-coal group Lock the Gate said Wollongong Coal's precarious financial situation and the mine's location in the drinking water catchment meant there was a higher than usual risk.

"If Wollongong Coal declared bankruptcy tomorrow, how would the cleanup of this mine under Sydney's drinking water catchment, on the edge of suburban Wollongong, be paid for?" he said.

"Taxpayers should be extremely concerned about the financial state of this company and the cost of cleaning up its mess."