The first allows the layout at Ashton Coal's underground mine to be modified and the diversion of Bowmans Creek.

Environment groups had lobbied against the project on the basis that it would pose a major risk to the Hunter River's alluvial plains.

But the department said the project would avoid a potential early termination of mining at the site and secure ongoing employment for 195 staff.

As part of the modification, Ashton Coal will be allowed to build a 1.7-kilometre diversion of Bowmans Creek in two areas and to change the mine layout to avoid mining under the creek.

It will also replace three of its narrow, or miniwall, panels with standard longwall panels to allow access to another 5.3 million tonnes of coal.

"The approved modification will allow the diverted Bowmans Creek and the longwall panels to be kept separate," Department of Planning Major Project Assessments executive director Chris Wilson said.

"This compares to the current [existing] approval which would have seen Bowmans Creek sitting across the mining panel layout."

A significant part of the area to be mined had been owned by the family of Camberwell resident Wendy Bowman, who said mining would destroy the fertile river lands that once supported four dairy farms.

"The whole area is criss-crossed with alluvial aquifers. It is some of the richest land in valley, which will be useless for anything other than grazing once mining occurs," she said.

The second approval, for the nearby Ravensworth underground mine, allows coal production to increase from five to seven million tonnes a year and the extension of four longwall panels.