NSW Planning Minister Anthony Roberts has confirmed the economic case for a controversial mine extension will be reviewed, amid calls for greater transparency in the assessment process.

Environment groups criticised the assessment of the Dendrobium coal mine extension in the NSW Illawarra region after emails obtained by the ABC revealed the Department of Planning advising South32 on its application for State Significant Infrastructure (SSI) status.

The SSI declaration makes Mr Roberts the consent authority for the project.

The declaration was made after the Independent Planning Commission (IPC) rejected the mine plan, claiming it would cause "irreversible" environmental damage.

Speaking the day after public comment closed on the revised mine plan, Mr Roberts backed the integrity of his department's assessment.

"We have an independent process in NSW," he said.

"We have the Independent Planning Commission, we also then have the department, and as of yesterday we've had over 500 submissions, and my department will go through those submissions.

"We are having the economic study reviewed independently, by an independent arbitrator, which is critical.”

'Independent expert'

A Department of Planning spokesperson said the economic case for the project would be subject to independent scrutiny. 

"All issues raised in that feedback will need to be addressed by the applicant and considered as part of our rigorous assessment process, along with independent advice," the spokesperson said.

The revised mine plan reduces the long wall mining area by 60 per cent, reduces surface water losses by about 78 per cent, and will reduce potential cultural heritage impacts.

Independent South Coast MLC Justin Field wrote to the Planning Minister last month asking him to ask the IPC to provide advice on the revised mine plan.

Mr Robert declined to commit to the request.

'Special treatment'

Former NSW Supreme Court judge Anthony Whealy SC believed the emails exchanged between the Department of Planning and South32 had affected the public's perception of the assessment process.

“I have no doubt that the perception has been given here, whether it is true or not, that special treatment has been given to South32 in this instance," Mr Whealy said.

Mr Whealy, who also worked as Assistant Commissioner on the Independent Commission Against Corruption, said there was a question about whether the Planning Department’s behaviour went too far.

“The issue here is whether the emails I have read cross that line and I would have to say it is a matter where minds may differ,” he said.

He said ultimately it was a calculated risk for the NSW government to overrule the IPC.

"The government in one sense acts at its own peril when it departs from a system that is set up for independent advice," Mr Whealy said.

"That is a matter for the government. If it becomes a tainted process well then that will affect the government when it comes to the next election."