Ofwat said more than 286,000 people were affected by supply problems over the three-year period.
An investigation by the watchdog found that South East Water failed to plan sufficiently, learn from incidents – such as the so-called Beast from the East wave of cold weather in early 2018 – and conduct analysis to discover the root causes of the problems.
Ofwat also said the company failed to maintain key infrastructure such as service reservoirs, boreholes and major pipes.
“All of these issues left the system more likely to fail during prolonged dry periods or freeze thaw events as we have seen in Kent and Sussex on multiple occasions,” said Ofwat.
South East Water has been contacted for comment.
