About 24,000 customers were affected by the water outage on Saturday, with 1,150 properties in the Pembury area getting supplies back on Sunday.
The company said 500 homes had water restored since 05:00 on Monday.
Matthew Dean, head of operations control at SEW, said Pembury Water Treatment Works had stopped working “due to a bad chemical batch”.
A new batch of the chemical was received on Sunday, he added.
SEW said it had delivered bottled water to more than 4,000 of its most vulnerable customers.
A SEW spokesperson added: “When the water supply returns, you may experience discolouration.
“If your water looks brown in colour, please don’t worry as it is not harmful.”