AGL Macquarie has been slapped with a $15,000 fine for water pollution at the Bayswater power station.
The NSW Environment Protection Authority confirmed the fine on Friday, saying it related to a discharge of fly ash slurry into a creek.
The Bayswater Creek leak came from a pipeline at the power station in April.
Contractors noticed a drop in flow rate as they monitored fly ash pumping.
They immediately stopped the pumps and investigated, discovering a hole in the transfer pipeline where the substance had escaped.
Staff notified the EPA and cleaned up the material.
EPA director regulatory operations north Adam Gilligan said the incident could have caused environmental harm given fly ash slurry covered vegetation and settled in the creek, which was running after rainfall.
"This incident was similar, but smaller in scale, to an event in 2019 for which AGL Macquarie has entered into an enforceable undertaking," he said.
"Under the enforceable undertaking, AGL Macquarie is continuing to assess and monitor the condition of the pipeline and has committed to replacing the aging pipeline."
AGL has sought development consent to replace the pipeline, and is working with the EPA to prevent future incidents.