Apex Energy is considering projects to extract gas from coal seams through fracking — a drilling operation which uses water and chemicals to fracture coal and pump escaping gas to the surface.

Nepean Greens member Peta Holmes said she was concerned about where the chemicals would end up.

"The gas and other toxins, including now-contaminated ground water, is then released," Ms Holmes said.

Lower mountains resident Suzie Wright, who was the Greens' candidate for Lindsay at this year's federal election, said the company would also have to build access roads into the catchment area.

"I grew up in Silverdale and my father worked for Sydney Water for about 30 years," Ms Wright said.

"We were never allowed to walk in the catchment because of the risk of polluting the water.

"How could they allow the building of roads into the area?"

A spokesman for Apex Energy referred the Star to a statement on its website.

"Apex Energy is undertaking an exploration program and has not prepared gas extraction plans for its exploration areas," it said.

"Apex has not developed a strategy for the Burragorang area and does not have a Burragorang planning application underway.

"If Apex does develop a plan it will be in strict accord with legislated requirements and follow consultation with all stakeholders, including the Sydney Catchment Authority."

Greens state upper house MP Cate Faehrmann called on the government to ban gas exploration at Burragorang, until it was proved safe.

"Coal seam gas exploration is a new industry and a lot of people are concerned about its impact on aquifers," Ms Faehrmann said.

"The community needs to be assured this is 100 per cent safe."