A Queensland coal mine is being investigated over allegations it has been operating outside its approved area.
Officers from the federal Department of Agriculture, Water and the Environment are looking into the potential breach at the New Acland coal mine in the Darling Downs region, west of Brisbane.
The Australian Conservation Foundation lodged the complaint with federal Environment Minister Sussan Ley this week, only to find it had been notified about the matter in December 2019.
The Queensland Department of Environment and Science has been investigating since 2018.
ACF campaigner Christian Slattery said investigations appeared to show the more than one kilometre-long West Pit fell beyond the boundary of the mine's footprint.
"It appears that parent company New Hope is pretty flagrantly flouting Australian environmental law," he told AAP.
But New Hope rejects the allegations.
"The mining of West Pit was considered in 2016 in the Land Court where the ... Department of Environment and Science, confirmed that the excavation of West Pit was not in breach of New Acland Coal's environmental authority," a spokesman said.
The spokesman said this was another example of "opportunistic green activist groups" using lawfare to attempt to delay the approvals of the next phase of the mine site.
"New Acland Stage 3 is a shovel-ready project that will create 187 new jobs within the first six months, 487 new jobs within 18 months and inject $7 billion into the Queensland economy," he said.
Mr Slattery said if investigators found the mine had been operating unlawfully, as ACF believed it had, "timely and appropriate enforcement" was necessary.
"It wouldn't be enough for them to simply receive a slap on the wrist," he said, adding New Hope's plan to mine five million tonnes of coal from the West Pit could generate $500 million in revenue.
"Our view is that any deterrent would need to be commensurate with the profit that New Hope have potentially unlawfully attained."
A spokesman for Ms Ley confirmed compliance officers were investigating the claims, as did Queensland's Department of Environment and Science.