The company wants to extract more "high-quality" rock from its Symonston quarry. It would be used to make concrete and asphalt for road and other infrastructure projects in the ACT and NSW.

But Boral first needs to extract "weathered rock" and top soil that sits above the more valuable commodity - and find somewhere to stockpile the leftover material.

It identified land next to its existing quarry for the so-called "overburden bund".

The chosen site encompasses 7.2 hectare of native vegetation, including box gum woodland, meaning Boral has been required to pass through a strict environmental approvals process.

An environmental impact statement prepared on behalf of Boral, and lodged with the ACT government in November, stated that if the project did not go ahead, the company would have to "modify" the existing quarry in order to access the valuable rock, which would cause greater environmental damage.

It canvassed the option of storing the materials at Boral's Jeir quarry, about 55 kilometres north of Mugga Lane, but that was not considered commercial viable.

To compensate for the loss of vegetation in Mugga Lane, it proposed using the NSW government's biodiversity credit scheme to support conservation in land north-west of Crookwell.

In a letter to the Department of Environment and Energy, Friends of Grassland president Geoff Robertson raised concerns about the arrangement, of which he said there was a "paucity of information".

"It is not clear why a site outside the ACT is being considered, nor whether alternatives inside the ACT were considered," Mr Robertson's letter stated.

"Is it part of a much larger protected grassly woodland area? What are its ecological values? What improvements will be made as part of the offset package to ensure no net loss across the landscape."

Despite the objections, the department approved Boral's proposal this month, on the condition it followed through on its promise to contribute to the credit scheme as compensation for the loss of native vegetation at Mugga Lane.

A Boral spokesman said it was now awaiting approval from the ACT Planning and Land Authority to begin construction work.

The spokesman said a 16-month construction phase would begin once approval was granted, and Boral had updated its environmental management plan.

The project would not result in an overall increase in the quarry's production levels, hours of operation or traffic movements in and out of the site.