Clarence Colliery will plead guilty after the environmental watchdog in New South Wales launched legal action over a coal mine collapse affecting the Blue Mountains World Heritage area.

More than 200 tonnes of coal fines have been removed from the Wollangambe River, after a wall collapsed at the Clarence Colliery near Lithgow on the state's central tablelands in July last year.

The Environment Protection Authority (EPA) has held 42 inspections at the site, and has begun prosecuting Centennial Coal with an alleged tier one offence in the Land and Environment Court.

Clarence Colliery, a subsidiary of Centennial Coal, said in a statement it would be entering a plea of guilty to the charges at the earliest opportunity.

The coal fines have been placed in bags and removed from the remote and rugged terrain by helicopter as part of clean-up efforts.

EPA acting chief environmental regulator Gary Whytcross said while the clean up had been satisfactory to date, the collapse was a major environmental incident.

"We have considered the matter over the last 12 months and come to the decision that a tier one offence is the appropriate matter to pursue in the Land and Environment Court," Mr Whytcross said.

"Environmental harm is always a key consideration of our decision about what charges to take, and certainly that's a matter that the court will look at as well.

"The fact that this was a sensitive environment certainly adds to the significance of the incident."

Coal fines have been removed from the rugged Wollangambe River region via helicopter. ( Supplied: River Deep, Mountain High )

The EPA said such an offence could result in a maximum fine of $2 million for a company.

In the statement, Clarence Colliery said it had fully cooperated with the the EPA and the Office of Environment and Heritage (OEH).

"In a court document, the EPA and OEH have acknowledged that Clarence's cooperation has been 'considerable and unusual'," the statement said.

"Prior to this incident, Clarence had a clean record, with no convictions for any environmental offence.

"Clarence and its parent, Centennial Coal, deeply regret the incident and apologise for its occurrence."

Mr Whytcross said clean-up work would continue.

"It's been a very, very careful clean up that's being conducted," Mr Whytcross said.

"It's a very remote area so it has to be cleaned up manually by hand and the people have been doing a good job there.

"It has been a very long process, but the careful process that's been employed has prevented the material from moving any further than where it actually settled."