Why a major Sydney river is bubbling

A mining company has admitted responsibility for gas bubbling up into one of Sydney's major waterways, but Illawarra Coal insists there's no threat to the Nepean River. Locals though aren't convinced.

On a stretch of the Nepean River beyond Campbelltown methane floats to the surface in several streams leaving an oily film.

Locals first noticed it a few weeks ago. The leak is on the Nepean River between Douglas Park and Appin, above the West Appin Longwall coalmine.

Residents say it's from riverbed cracks, caused by mining under and near the river and it isn’t as worrying as what you can't see.

"We only know it's there because of the water, but I'd say it's coming out over the landscape the whole of the district," Mike Wiliiams from Lock The Gate said.

The bubbling gas is three kilometers from the mine's main entry, but it’s longwall mining, with tunnels running under a huge area of land.

The Appin mine is operated by Illawarra Coal, a subsidiary of BHP Billiton.

While Illawarra Coal says it doesn't mine directly under the river, it concedes there is what it called 'minor gas bubbling, but it's non-toxic, and within approved limits.