The NSW Environment Protection Authority (EPA) has begun legal action action against the owners of the Vales Point Power Station over allegations improper maintenance led to a large-scale fish kill last year.

The environmental watchdog alleges Sunset Power International Pty Ltd, who operate under the trading name Delta Electricity, failed to maintain their chlorine dosing plant on the Central Coast that led to a faulty valve.

This allegedly caused a discharge of concentrated sodium hypochlorite to leak into Wyee Bay, Mannering Park and resulted in the death of about 15,000 fish.

The case, which was filed on Wednesday, will now continue in the NSW Land and Environment Court.

NSW EPA chief executive Tony Chappel claimed the EPA’s comprehensive investigation found that Delta “could have prevented the fish kill if they had adequate equipment and processes in place”.

“Alongside the community, we expect Delta to meet high standards given the proximity of the Vales Point Power Station to Lake Macquarie,” Mr Chappel said.

“We’re pleased that Delta have upgraded their systems and the water quality in the lake has returned to normal levels.”

The breach of section 64 of the Protection of the Environment Operations Act 1997 (POEO Act) carries a maximum penalty of $1m.

Lake Macquarie MP Greg Piper welcomed the legal action and said the fish kill had been “extremely distressing” to the Mannering Park community.

“As this matter is currently before the courts I ask you to be careful with your comments, but I am very pleased to see that the EPA has taken such serious action following their comprehensive investigation into this incident,” he shared in a comment made on Facebook.

A Delta spokesman said the company took these “allegations seriously and is currently reviewing the details of prosecution which have now been provided by the EPA”.

“As the matter is now before the courts, it is inappropriate for Delta to comment further at this time,” he said.

Vales Point Power Station produces about 4 per cent of the electricity used in all states and territories except for Western Australia and the Northern Territory.