But opponents condemned the coal miner's social impact assessment, telling the Independent Planning Commission (IPC) the company wouldn't pay any promised taxes and would help destroy the planet.

Whitehaven director David Ellwood was the last of 65 people to have their say on the project.

Commissioner Mary O'Kane said it was "striking" that the project had widespread support within the town community of Narrabri, but many landholders had objected to the project.

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Many farmers told the IPC they were concerned the expanded mine would affect their water aquifers.

"It's a difficult question to answer..." Mr Ellwood said.

"This is something new to these landholders and it's something that they've got to learn.

"A majority of the existing landholders surrounding the mine aren't making any submissions. These are a new group of landholders who are further away from the existing mine, haven't been involved in the mine in the past, and this is something they're just starting to come to grips with."

If approved, the mine extension would allow Whitehaven to prolong the life of the existing Narrabri Underground Mine to 2044, an additional 13 years.

The Department of Planning Industry Environment recommended imposing more than 40 pages of restrictions on the project, but ultimately decided it would provide a net benefit of about $599 million, accounting for environmental costs.

The department's net benefit figure came under repeated attack by opponents during the hearing.

Earlier on Friday, academics Alison Ziller, Prof Ian Lowe and former Chief Scientist Penny Sackett condemned the project's effects on the world's environment.

Scone-based objector Sue Abbott told commissioners they would be furthering the climate change-driven destruction of the planet if they signed off on the project.

Climate Energy Finance director and independent financial expert Tim Buckley told the panel the coal industry is in terminal decline, and Whitehaven wouldn't pay a dollar of a forecast $177 million in federal corporate taxes owed.

Whitehaven earned a cash tax refund of $22 million over the last decade, he said.

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"The idea that they're going to pay any corporate tax in Australia, history would suggest the opposite," he said.

Whitehaven's Mr Ellwood said the project had been carefully designed, was in line with the state government's strategic statement on coal and was in the public interest.

Written submissions will be accepted until 5pm on Friday 25 February 2022.

The IPC will now retire to consider both public and written submissions regarding the project, before deciding whether to approve it and under what conditions to do so.

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