Southern Water has been fined £24,000 and will pay costs of £33,218 after draining excess waste into a stream.
An investigation by the Environment Agency found the company discharged more treated sewage than permitted from its Tunbridge Wells North wastewater treatment plant into the Somerhill stream.
The environmental permit conditions set by the Environment Agency to support a healthy river for wildlife and recreation were exceeded between July 2013 and July 2014.
Southern Water pleaded guilty to the charges and said it has already spent £360,000 on improving the site, with a further £6 million planned for the future.
David Willis, Environment Manager at the Environment Agency, said: “We take these incidents very seriously and do everything within our powers to safeguard the environment and people that may be affected.
“We expect companies to take all necessary actions to comply with the conditions set out in environmental permits and welcome Southern Water’s commitment to spend a further £6 million on planned improvement works.”