AGL, which operates the Bayswater Power Station near Muswellbrook, admitted responsibility for the leak, which happened during a re-certification of a bulk ammonia tank.

The tainted water washed into a stormwater drain before entering the creek via a licensed discharge point. However, the pH limit range was exceeded.

The company reported the incident to the EPA on April 9.

Despite acknowledging that the damage had luckily been minimal, EPA's Hunter region manager Adam Gilligan urged AGL to up its vigilance and improve its environmental management systems.

"Our investigation has found that the environmental harm was minimal mainly due to the dilution of the ammonia in the water and the relatively small volume which was released.

"However, this is the third chemical spill incident that AGL Macquarie is responsible for this year, which suggests a lack of appropriate environmental controls.

"It is for this reason that the EPA has issued the Company with a penalty notice for the offence of failing to comply with an environment protection licence condition.

"This incident is a lesson to all companies to plan and manage any water related process with care and consideration of the potential risks to the environment," said Gilligan.