The Protect Our Water Alliance group (POWA) said they had 50 people in attendance at a meeting on Thursday night in Wollongong

POWA spokesperson Rada Germanos said South32's plans were putting the region's water security region at risk.

"This comes at a time when our dam levels have fallen below 50 per cent, level one water restrictions are in place, and the desalination plant is in use," she said.

"It is bewildering to think that the government might approve these expansions that will drain more and more water from our catchments every day."

Ms Germanos said independent scientific evidence showed groundwater losses from mining in the catchment are already about three million litres of water a day.

"These are set to almost triple with this proposed expansion to the Dendrobium mine," she said.

"We as a community cannot let this expansion go ahead, given its grave threat to our water security.

"We're calling on the NSW State Government to halt all mining in the Greater Sydney water catchment."

The Dendrobium mine expansion proposal is on exhibition for public submissions until September 18.

For its part, South32 referred to expert evidence it had commissioned to assess the environmental impact of the project.

It has previously said this mine and Appin would close if this expansion is rejected.

"We have submitted a development application to continue mining activities in Dendrobium's next two underground mining areas, within the existing mining lease," a spokeswoman said.

"The mine has development consent until 2030, however current mineable reserves will be depleted by 2024.

"The supporting EIS [environmental impact statement] is informed by years of expert research into the environmental, social and economic aspects of the project and has been developed through extensive consultation with our stakeholders.

"We take our environmental responsibilities seriously and understand the sensitivities of working within the Metropolitan Special Area.

"The EIS contains specialist environmental studies and thoroughly assesses all environmental aspects of the project in the context of its location.

"Our extensive water monitoring program helps to inform our future mine planning and we share the data with the Department of Planning and Environment and other government agencies.

"Approval of the project will sustain the employment of around 400 people, create an additional 100 jobs and ensure the supply of metallurgical coal to local and overseas customers until 2048. It will also create 200 additional jobs during the construction period, and continued business opportunities for local suppliers."