The EPA delivered the clean-up notice after farmer Ros Druce discovered thousands of small expandable polystyrene balls (EPBs) in Back Creek, near Boggabri.

On February 10, officers identified that the EPBs had been discharged from the mine into the creek, while more EPBs were found on a property seven kilometres downstream of the mine.

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The EPA had previously inspected the Maules Creek Coal Mine in August 2019 and found that "EPBs were not being contained within the explosive depot"; and confirmed "verbal instructions were given to Maules Creek Coal Mine to rectify the situation and contain these EPBs within the depot".

"Whitehaven takes any instance of non-compliance with obligations seriously and is disappointed about the release of a number of EPBs from an explosives depot following February's heavy rain events," a Whitehaven spokesperson said.

"We are working closely with our on-site explosives contractor and the NSW EPA to ensure full compliance with the clean-up notice."

Whitehaven has been ordered to carry out a number of steps to rectify the situation, including a full clean out of one its dams.

Part of those steps also include ensuring no polystyrene balls are located in any structure designed to carry water - such as sub-drains, stormwater drains and stormwater dams - at the premises.

The coal company has until March 25 to comply and also has to pay a fee of $563 in administrative costs.

However, Ms Druce said the EPA needed to go further.

"The EPA needs to get tough with this company - simply issuing a cleanup notice is not enough when Whitehaven was warned more than seven months ago," she said.

"Whitehaven knew about it, they were warned about it, and it still happened."