Seven open-cut mine sites are under investigation for illegally burying tyres in a effort to avoid recycling costs.
An Environment Protection Authority spokesman said mine records were currently being examined to determine if tyres had been buried without consent.
The investigation was triggered by farmers and environmental groups which raised the alarm about the impacts of buried tyres.
Whitehaven Coal recently received formal approval to bury hundreds of giant tyres at its Werris Creek and Tarrawonga mine sites, however, the company's annual review for Tarrawonga states tyre disposal has been taking place on site since 2014.
The EPA knocked back a proposed amendment by Whitehaven to bury its tyres at Maules Creek in 2019.
However, Whitehaven's Maules Creek Mining Operations Plan states "end of life mine tyres are also collected, recorded and placed within the overburden emplacement at a suitable depth to ensure appropriate coverage under the final landform."
Maules Creek resident Ros Druce said the impact of buying old tyres needed to be taken seriously.
"I hope that the Environment Protection Authority takes into account the cumulative effect of tyre brial in the Leard State Forest - these tyres are buried deep underground and could have a disastrous impact on our groundwater and the surrounding environment," Ms Druce said.
Lock the Gate Alliance NSW spokesperson Georgina Woods said the alleged behaviour was alarming.
"The Maules Creek and wider Namoi community has fought long and hard to uncover Whitehaven's long list of environmental breaches," she said.
"It's time the NSW Berejiklian Government got serious with Whitehaven and threw the book at this recalcitrant company."
Tyres contain chemicals and heavy metals that leach into the environment as the rubber breaks down.
Research conducted in the US shows leaching affects the soil around the old tyre.
If toxins get into water in the soil, they can be transported to other locations and potentially enter the food chain.
Section 143 of the Protection of the Environment Operations Act 1997 requires that tyre waste must be transported to a place that can lawfully accept it.
There are also requirements to track loads of waste tyres greater than 20 tyres, or 200kg within NSW.
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