The Duralie coal mine is due to get deeper, if given a green light by NSW state government.
The Hunter Valley mine near Gloucester will increase in depth, but also expand the surface development by 2.5 kilometres, and remove a pillar between two pits.
According to an environmental assessment the development will be required to ensure efficient extraction of coal while maintaining geotechnical stability of the pit walls.
The assessment states that waste emplacement will be constrained if the modifications do not go ahead.
It also said that reserves already approved for extraction until 2019 in the Weismantel and Clareval pits would have to be forgone.
The proposed modification has been estimated to make no changes to maximum annual coal production, waste rock extraction, the seams mined or size of workforce.
Duralie mine is owned by Duralie Coal, a subsidiary of Yancoal Australia.