Ballymoney water testing to continue this week
Three thousand households in Ballymoney who are having to boil water before use have been advised to continue doing so.
The problem started on Tuesday and Northern Ireland Water (NIW) had hoped it would be resolved on Friday, but it has continued over the weekend.
It said on Sunday tests were showing an "improving picture" but that the boil water notice would continue".
NIW said this would allow for two days of tests before it could be lifted.
Northern Ireland Water's head of quality and compliance, Dymphna Gallagher said: "The problem is taking longer than expected to resolve due mainly to the length and complexity of the local water distribution system; it extends to over 88 km (55 miles) in length and is mainly rural in nature.
"Northern Ireland Water is making every effort to restore the quality of water to the normal high standard as quickly as possible, through additional chlorine dosing and extensive flushing of the local water distribution system."
NI Water has apologised for the inconvenience caused to customers and will continue testing samples until the water quality returns to normal.
It has provided a list of postcodes affected by the problems.
It will notify customers by card as well as through news and other media outlets.