It has become the latest miner in this region's coalfields to reduce its resource extraction aspirations in order to try and navigate environmental impacts from underground collieries.

Operating company SIMEC said its amended plan for the proposed southern extension of Tahmoor would reduce subsidence effects as a result of reducing the height and the width of longwall panels to be mined.

It also said there would be a "substantial reduction" in the extent of mining underneath existing homes, as well as reduced impact on shale sandstone transition forest above ground.

Read more: Jobs secured as Tahmoor mine deal done and dusted

The extension would allow employment to continue for nearly 400 people for 13 years, with an extra 175 jobs added during the first two years of construction.

Up to 4 million tonnes would be extracted each year and a new rejects emplacement area built.

SIMEC executive managing director Matt Reed said there had been no significant objections from government departments, but the company had worked to address submissions made from agencies and the public.

"The revised plan contains significant environmental impact improvements while also preserving, if not improving, important local area economic and community support benefits," he said.

"We believe our plan for continuation of the mine's operations for a further 13 years can make an important contribution as the [Wollondilly] shire and the community work to overcome the impacts of the recent bushfires.

"If approved, the mine extension will be a major source of secure employment and related economic stimulus valued at around $160 million for the local shire over the 13 years of operations."

The mine extension plan has been opposed by some in the community, including the National Parks Association of NSW, which said "catastrophic impacts" to Myrtle and Redbank creeks remained unremediated after 14 years of mining.

The NPA also said scientific studies of the impact on the Thirlmere Lakes would not be complete until later this year, so decisions should not be made on "incomplete" knowledge.