Delta Coal aims to combine its two mines in Lake Macquarie and extend operations to 2029.
The plan, which will protect 390 jobs and earn $36 million in royalties for the NSW government, has attracted community concerns over the health of the lake and people.
The NSW Department of Planning put the proposal on public exhibition on Friday.
The closing date for submissions is December 16.
Environmental Justice Australia lawyer Jocelyn McGarity said if the project attracts 50 objections, the plan will be referred to the Independent Planning Commission for "more rigorous assessment and community participation".
Keep Lake Macquarie Clean member David Ransom said the lake often appears "idyllic and tranquil" on the surface.
"But underneath is a network of mines providing coal for a power station on its banks and, with it, the threat from vast hidden dumps of toxic coal-ash and polluted air," Mr Ransom said.
He said the group feared the plans could "further adversely impact the water, ecosystems and our broader community".
Delta is seeking to consolidate its "Chain Valley Extension Project" and Mannering Colliery "into one new development consent".
Delta spokesperson Steve Gurney said they had "operated as one mine" for years.
He accused "anti-coal activists" of having "one objective" - to "shut down coal-fired power stations".
"Delta Electricity takes its environmental responsibilities seriously," he said.
The company's environmental impact statement said the plans would provide ongoing job opportunities "across the two operations" for about 390 full-time employees. It estimated that the plan would have a net benefit to NSW of "at least $89 million". The NSW government would receive an estimated $36 million in royalties "over the life of the project".
The impact statement said about 13.4 million tonnes of coal was planned to be extracted from 2023 to 2029.
This would mean an extra "9.5 million tonnes" of additional coal being extracted above existing approvals for the two mines, which run to December 2027.
Ms McGarity said this means "another 9.5 million tonnes of coal to be burned" at the Vales Point plant.
The mine plan coincides with the planned 2029 closure of the coal-fired plant, although there has been talk about an extension. Mr Ransom said the community would like more transparency around the closure date.
Ms McGarity said the plant "pumps toxic pollution into the air we breathe and causes serious health conditions, including asthma in children, chronic lung problems and low birthweight in newborns".
"Additionally, this proposed mine expansion is to occur underneath Lake Macquarie. Given the two recent fish kills in the lake, it's important that this proposal undergoes the highest level of scrutiny."
Mr Gurney said EPA analysis "consistently shows that air quality on the Central Coast and Lake Macquarie is very good and amongst the best in NSW."
