The current planned 698-hectare expansion of Rio Tinto's Mt Thorley-Warkworth coalmine gained a recommendation for approval from the Planning Assessment Commission in March after courts twice knocked back earlier versions of the plans.

The commission said Bulga residents should consider relocating the town as one option to minimise the noise and dust of a mine that would encroach to about 2.6km from the town,  20km south-west of Singleton. (See map below, with Bulga located in the lower left-hand corner, and new mine areas marked in red).

However, a secret June 2014 report by Cumberland Ecology ordered by Rio and obtained by Fairfax Media, show the company has scoped out a further 1370 hectares on both the western and eastern flanks of the current mining plans. (See map below, with assessed areas in red.)

The report was prepared for the Baird government's Upper Hunter Strategic Assessment program to streamline future mine approvals. As Fairfax revealed in February, the project aims to determine the cumulative impacts on plants and animals of 16 proposed new and expanded coalmines covering about 45,000 hectares in the region.

Details of Rio's future intentions for the Warkworth mine, though, have not been made public until now.

This latest expansion, if approved, would bring the mine "to right outside the village door", said John Krey, Bulga resident and spokesman for the Bulga Milbrodale Progress Association.

Rio Tinto has been "trying to play it very low key", with the company leaving its intentions unclear for "undesignated" land adjoining the current mine continuation project, Mr Krey said.

Not-so permanent offsets

The miner's current expansion has been dogged by legal troubles, including over the clearing of endangered Warkworth Sands woodland vegetation that had been previously set aside as a permanent offset by Rio Tinto when it won approval in 2003 for an earlier expansion.

The scoping study indicated at least as much endangered vegetation is at risk, with 589 hectares of the threatened Central Hunter Grey Box - Ironbark woodland in the path of bulldozers.

"The impact is far, far greater than originally envisaged by the Planning Assessment Commission," said Georgina Woods, a spokeswoman for the Lock the Gate alliance. "Certainly the PAC would be aware of it."

The expansion would also sever the Wollombi Brook, a tributary of the Hunter River and an importance cultural site for the Wonnarua people indigenous to the area, Ms Woods said.

No approval to mine

A spokesman for the miner declined to comment on whether Rio Tinto planned future mining of the 1370-hectare area.

"We cannot mine in this area because we do not have approval under the NSW planning system to do so," the spokesman said. "Our focus is on securing approval for the applications we have before the NSW Government to continue operating Mount Thorley Warkworth for the next twenty years, which do not include this area."

The Rio spokesman also denied  residents would be forced to move.

"We are not seeking to relocate the village of Bulga," he said. "In fact we are committed to working with community members to ensure there is a strong future for Bulga."

A spokesman for Environment minister Mark Speakman said: "Because the assessment is being undertaken by the [Office of Environment & Heritage] and deals only with biodiversity, it cannot and does not purport to specify where mining may or may not ultimately be permitted."

"Indeed, the assessment does not necessarily relate to projects that are even proposed,"  the spokesman said.

Bulga residents are planning to hold a protest rally in Martin Place on Monday morning and present the government with a petition containing thousands of signatures opposed to the expansion plan recently recommended for approval by the PAC.