Environment officials questioned use of land government already owned as offset for western Sydney airport

Federal environmental department officials questioned the credibility of a government plan to use heritage-listed land it already owned as the main environmental offset for the western Sydney airport.

Documents obtained by Guardian Australia under freedom of information laws show officials asked the federal infrastructure department to justify the use of Defence Establishment Orchard Hills to offset the destruction of more than 100ha of critically endangered Cumberland Plain woodland and other habitat.

Defence Establishment Orchard Hills is a defence-owned site in western Sydney that is used to store explosives and run training exercises.

It is also one of the largest remnants of endangered bushland in western Sydney, with 1,370ha listed on the commonwealth heritage register for its natural values – specifically that it is almost entirely vegetated with critically endangered Cumberland Plain woodland and Sydney Coastal River Flat forest.

In 2018, the federal government granted approval for this site to be used as the main offset for the western Sydney airport.

But new documents, including comments on the draft offset plan, show officials noted the site already had heritage protections and other environmental management that meant the risk it could be lost in future was “close to zero”.

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Additionally, they asked the infrastructure department to explain how the proposed management of the site as an offset would significantly improve the quality of the habitat, given that it was already being managed by defence for its environmental values.

In a response that conservationists have described as “farcical”, infrastructure wrote that in the absence of the offset there was a risk the site could be entirely lost to fire or development.

It also wrote that its proposed management actions would significantly improve lower quality areas of habitat on the site.

“There is a risk that the threats that are currently functioning to degrade the community would increase in severity to the extent that the entire local occurrence would be lost without active management,” the department wrote in comments that also appear in the final offset plan.

“Notably there is a risk of catastrophic wildfire given the substantial areas of native blackthorn scrub at the site.”

Defence Establishment Orchard Hills has been managed for fire for years, given its use as an explosives storage facility.

It also has other plans in place, including a biodiversity management plan.

The government’s own heritage listing states that when the condition of Orchard Hills was assessed in 2000, fire had been “excluded from the site for over 50 years due to security of the munitions dump”.

A defence spokesperson told Guardian Australia there had been six fires in the area since 2000.

They said the site had a bushfire management plan that defence updated after a major review every five years and mitigation work also occurred every year to address seasonal fire requirements.

Brendan Sydes, of the Australian Conservation Foundation, said it was farcical for the infrastructure department to argue the site was not already properly managed for fire.

He said the organisation also did not accept that the environmental management that already occurred under the heritage listing was so lacking it justified the use of the site as an offset.

“It is commonwealth land, managed by a commonwealth entity, protected as commonwealth natural heritage under commonwealth law – you can’t get much more secure than that,” he said.

He said the offsetting system under the Environment Protection and Biodiversity Conservation Act was fundamentally flawed and allowed for unique ecological communities to be continually chipped away at.

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“This is an example of a dodgy offset that the government lets through under the current EPBC Act,” Sydes said.

Paul Rymer, a senior lecturer in the Hawkesbury Institute for the Environment at Western Sydney University, sat on an expert group that provided advice to the infrastructure department when it was preparing the offset plan.

He questioned the department’s claims the ecosystems on the site were at risk of being entirely lost to fire or development in the absence of the offset.

“Those risks are not real. There was always protection on the site, making development difficult,” he said.

Lisa Harrold is the president of the local Mulgoa Valley Landcare and was another member of the expert group.

She opposed the use of the Orchard Hills site, describing the decision as “an easy way out” of offsetting obligations.

“The notion that the entire conservation area at Orchard Hills might be completely and permanently destroyed by wildfire when already protected by a robust and military style fire protection plan is absurd,” she said.

“The notion that these conservation lands might be entirely destroyed by development when already protected by the EPBC Act and a commonwealth heritage listing is misleading.”

A memorandum of understanding between defence and the infrastructure department states the planned offset activities would include weeding, erosion control, revegetation, and the provision of fallen timber and nesting hollows for wildlife.

A tender has been advertised for the work but a contractor has not been chosen yet.

A spokesperson for the infrastructure department said the offset funding would substantially improve the site over the next 20 years.

“Around half the area is degraded former farming land, rubbish dumps and previously developed areas that will be rehabilitated to improve the values,” they said.

“The Orchard Hills site allows for meaningful restoration activities in proximity to quality remnants that will combine to provide a significant connected landscape of remnant and regrowth vegetation within and outside the offset area.”

An environment department spokesperson said its comments on the offset plan were provided to infrastructure as part of the standard EPBC Act assessment process.

“In response to those comments, infrastructure amended the plan to highlight the significant pressures on native vegetation, including Cumberland Plain Woodland,” they said.

“Infrastructure also expanded the [offset plan] to clarify their assessment of habitat quality at both the impact and offset site.”

The spokesperson said a 2020 audit had found the department was complying with the conditions of its environmental approval.