MACH Energy, the operator of the Mt Pleasant mine, wants to extract an additional 247 Mt (million tonnes) of coal from the site near Muswellbrook and extend its operations until 2048.

In June the NSW Department of Planning and Environment (DPE) recommended the project be approved subject to a number of environmental conditions, with the final decision now in the hands of the IPC panel.

Nic Clyde, the NSW coordinator for Lock the Gate, said the planned expansion of the Mt Pleasant mine "makes no sense" and should be refused.

"The damage to human health caused by air pollution and the legacy of a vast, unfilled mine pit filled with toxic wastewater will cost Hunter residents dearly," Mr Clyde said.

"This coal mine will also make a massive contribution to global heating. This will cause more extreme weather events like the recent east coast floods and will harm Hunter communities."

Over two days of public hearings on July 7 and 8, the IPC heard from a number of scientific experts, including former Chief Scientist of Australia Professor Penny Sackett, who raised concerns about the project's social and environmental impacts.

Professor Sackett told the IPC hearing if the project was approved, emissions from coal mined at Mt Pleasant and elsewhere in the Hunter Valley "would equal two per cent of the world's remaining carbon budget to hold global heating to 1.5 degrees Celsius".

"Another way to say it is that the emissions from the combustion of this coal would equal all of Australia's direct emissions from all sources over the whole period from 2022 to 2050, just the emissions from the Hunter Valley coal once it's combusted," Professor Sackett said.

"That's why what the IPC does, the decision it makes on this project is so vital."

Dr Gabriel da Silva, an associate professor at the University of Melbourne, told the hearing approving the Mt Pleasant expansion would "inevitably result in increased air pollution in the Hunter".

Dr da Silva said new research showed exposure to fine particulate pollution (PM2.5) like that caused by coal mining is more harmful than previously understood.

"If you look at Muswellbrook as an example, it already experiences significant exceedances of the standards that we set as the country for PM2.5. It does this on a daily basis and it does this on an annual basis," Dr da Silva said.

"The latest studies are showing we can't actually work out where a safe limit for exposure to PM2.5 is. There's no known safe exposure level.

"The Mt Pleasant expansion could add 5 or even 10 micrograms per metre cubed of PM2.5 in Muswellbrook's air on any given day. And we know from the latest science that this is a large burden on its own to the health of the, for those people that live there.

"And because of all of this, it's my expert opinion that the mine shouldn't be allowed to expand."

Community groups in the Upper Hunter also spoke against the project at the IPC hearing.

Wendy Wales, a member of Denman Aberdeen Muswellbrook Scone Healthy Environment Group Inc, said the Muswellbrook community was often conflicted when it came to coal mining, but there was an expectation the government would intervene to protect their health and the environment.

"In the time I have lived in Muswellbrook, I have experienced the cumulative impact of coal mining, particularly of the deterioration of landscape and air quality, to the point that Muswellbrook postcode was declared to have the worst air quality in New South Wales," Ms Wales said.

Despite the environmental concerns raised during the IPC hearing, public submissions on the Mt Pleasant project received so far are overwhelmingly in favour of the project's approval.

Of the 842 submissions received at the time of publication, 671 were in support of the project with 151 opposed, while 20 submissions were categorised as 'neutral'.

Many of those speaking in support of the project at the IPC hearing spoke favourably of MACH Energy's engagement with the local community and the positive impacts of continued employment opportunities in the region.

Michael Kelly, the president of the Muswellbrook Chamber of Commerce and Industry, said the Muswellbrook economy was largely dependent on coal mining.and said MACH Energy was seen as a "responsible corporate citizen" and "an integral part of the Muswellbrook community".

Steve Fordham, the founder of Blackrock Industries, an Indigenous owned mining services company based in Muswellbrook, told the hearing the IPC should consider the positive impact MACH Energy had made on the local community, particularly through its support of employment programs for incarcerated Indigenous men.

"This pit is so much more than just a mining company. It is our livelihood, this is our dream and it's who this town is and MACH Energy goes above and beyond everyday to make that apparent," Mr Fordham said.

Following the deadline for public submissions on the Mt Pleasant expansion on Wednesday, July 20, the IPC panel is expected to determine whether to approve the project by mid to late August 2022.

Public submissions on the Mt Pleasant Optimisation Project can be made via the IPC website.