Three thousand households in the Ballymoney area, which have to boil water before use, are likely to have to continue to do so over the weekend.
The problem started on Tuesday and Northern Ireland Water had hoped it would be resolved on Friday.
NI Water said it was "working continuously on the ground to resolve this problem" and was carrying out two sets of tests per day.
However, it said there had to be two days of clear tests to lift the notice.
NI Water's head of quality and compliance, Dymphna Gallagher, said they "were continuing to flush the system and increase chlorine levels".
"Further tests today have shown an improving picture.
"We will have a further set of results later today and tomorrow morning, however, we must have two clear days of tests to lift the notice and therefore it is likely to remain in place over the weekend."
At Carrowreagh Primary School near Ballymoney, the children have been drinking bottled water since Wednesday morning, when one of the teachers heard about it on the radio.
Teacher Arlene Graham said: "Teachers now are just reminding children all the time 'don't use the water in the sink, just come to me and get a top up from the bottle at the front of the classroom'."
Bottled water has also been supplied to people on a critical care list.
'Not good enough'
The MP for North Antrim has said it is "simply not good enough" that 3,000 households still have to boil their water before use.
Ian Paisley jr, the MP for the area, said: "I think the water service need to get their act together sooner rather than later.
"After the calamity of last winter in Belfast and the board's ongoing problems with their chief executive, the last thing they need is another pitfall like this."
The problem was caused when the system became contaminated after soil got into it during work to replace water pipes.
It has meant thousands of litres have had to be flushed out.
Customers in Bendooragh, Balnamore, Finvoy as well as the Ballymena Road and Bann Road areas of Ballymoney town have been affected.
Water used for bathing, showering and washing clothes is not a concern.
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 then notify customers by card as well as through news and other media outlets.