The company is said to have caused harm from "a pattern of repeated incidents over several years"
Southern Water has been fined more than £7.1million after admitting 13 offences involving illegal sewage discharges off the Kent coast.
The pollution incidents took place at wastewater pumping stations in Margate and Broadstairs between 2019 and 2021.
The water company pleaded guilty at Medway Magistrates' Court last April before the case was sent to Canterbury Crown Court for sentencing.
Following a two-day hearing on Thursday and Friday, Mr Justice Johnson ordered Southern Water to pay £7,127,083.
The judge described the harm caused as stemming from "a pattern of repeated incidents over several years" rather than an isolated occurrence.
Of the 13 convictions, nine related to incidents where untreated sewage was discharged into coastal waters off Kent.
Three further counts concerned the company's failure to alert authorities about the discharges within the required timeframe, breaching conditions of its environmental permit that mandate notification as soon as practicable and within 24 hours of any warning.
The remaining offence involved Southern Water operating without a backup pump at its Margate facility for more than a year, from 27 July 2019 until 4 October 2020, violating permit requirements.
This latest prosecution follows a £90million penalty imposed on the company in 2021 by the Environment Agency for nearly 7,000 separate incidents across Hampshire, Kent and Sussex.
Mr Justice Johnson stated there were "overall serious failures" by the company, noting that Southern Water understood the critical importance of maintaining resilience systems and equipment at these locations.
"It was well aware of the potential for equipment to fail and for the essential need for robust maintenance and testing procedures," the judge said. "That is because of the dozens of previous occasions on which that had happened."
He told the court that the combined offending resulted in "serious degradation of environmental quality, significant interference with public amenity, potential risk to public health and damage to the reputation of an important coastal community."
The judge highlighted Southern Water's extensive criminal record as "an exceptionally serious aggravating factor" in determining the sentence.
The company has accumulated 174 previous convictions, with guilty verdicts recorded annually from 1999 through to 2016, and most recently in April this year.
Mr Justice Johnson described the firm, which generates annual revenue between £800million and £1billion, as having demonstrated "a protracted history of non-compliance with its legal obligations, and a repeated pattern of inadequate staff training, insufficient investment in the infrastructure and a failure properly to maintain equipment."
He added that Southern Water had failed to heed warnings from the courts, including from the resident judge and the Lord Chief Justice.
