The company had said supplies were back for 12,000 customers on Tuesday.
Its head of water quality, Dr Neil Hudson, said after recharging their network in Tunbridge Wells they saw a “recurrence of the water quality issues” which caused the initial shutdown of the water treatment works in Pembury.
“Because of this, we are currently unable to produce drinking water from the Pembury treatment works that meets the strict regulatory standards required for consumption,” he said.
He urged residents to use water only for flushing toilets or showering, and said water could not be “used for drinking and cooking without boiling first”.
The problem, which began on Saturday, has seen customers in Tunbridge Wells, Pembury, Frant and Eridge experience a loss of water or low pressure.
The company blamed a “bad chemical batch” as the source of the problem.