The $3.6 billion project was subject to seven days of public hearings last month, with a list of over 400 speakers on the scheme the longest in NSW history. Over 10,000 people and groups made a written submission to the IPC.

But opponents of the project cried foul this week, after proponent Santos released new economic modeling for the controversial coal seam gas project.

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A Santos spokesperson said the company is "disappointed that a decision on the project has been delayed, however, no one can now say they have not had ample opportunity to have their say on the project".

Australia Institute Principal Advisor Mark Ogge demanded extra time to consider the new modeling and make a new submission based on the new information.

The IPC has granted his request, it announced on Friday.

The project was to be determined by the IPC by September 4. They now have until September 30.

The Commission will reopen public comment on the new information it had received from the Department and Santos until August 21.

New public comment can only be made on Santos' submission issued on August 10 and a document containing answers to IPC questions by the Department of Primary Industry and Environment.

Citing the level of complexity of the issues under consideration and the high level of public interest in the project the IPC successfully appealed to the Secretary of the Department of Planning, Industry and Environment for an extension of their determination deadline.

The plan to drill 850 coal seam gas wells into an area in and around the Pilliga forest has been controversial for decades.

Supporters of the project said it would create jobs and economic opportunity in the Narrabri community, and could be done safely.

But opponents claimed the mining project would risk damage to a local water aquifer, create bushfire risk and threaten the world through climate change.

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