The extra cost of keeping sewers working, as the South East experiences the worst series of storms in over 10 years, is running at more than £70,000 a day.
Southern Water has a fleet of tankers as well as mobile pumps working round the clock throughout key points in the region to deal with the problem caused by local drainage networks and flood defence systems being overwhelmed.
While those drainage systems are the responsibility of others, such as local councils and the Environment Agency, Southern Water maintains a duty to keep the sewer network operating to prevent sewage overflowing and stop customers being unable to use their toilets and sinks.
Director of Communications Geoff Loader said: “When drainage systems and flood defences, which are the responsibility of other agencies, are unable to prevent flooding our sewers are at risk.
“The flood water gets into the sewers and they are simply not there to carry this water. Therefore, we have to use tankers and mobile pumps to try to keep them operating and dealing with the wastewater from homes and businesses - the job they are designed to do.”
In some areas the water table rose 10 metres in five days, causing underground springs to reach the surface, overwhelm drainage networks and flood the sewers. In others parts of the region overflowing rivers have caused a similar issue.
Mr Loader said: “In the past year alone, we have spent more than £4 million to seal sewers and manholes to prevent flood water getting into them. More of this work is planned but it is important to realise this will deal only with a symptom, and not the cause, of the flooding.
“That is for other agencies to look at and we are committed to supporting their plans to prevent this flooding in the future, particularly in vulnerable areas such as St Mary Bourne and Hambledon in Hampshire and along the Nailbourne in Kent.
“In the shorter term, we must ensure our customers can continue to use their wastewater services and this is our priority.”