Irish Water says it will be Tuesday at the earliest before a boil water notice for customers of the Leixlip Water Treatment Plant is lifted.
More than 615,000 people in parts of counties Dublin, Kildare and Meath have been affected by the advisory, which has been in place since Monday evening.
The managing director of Irish Water has appealed to the public to have patience with the company.
Speaking on RTÉ's Six One News, Niall Gleeson said it will take time to fix the issues. He also said that boil water notices may be issued to other parts of the country in order to bring water treated at the treatment plant to a satisfactory level.
A test carried out on the water quality today returned satisfactory, but Irish Water says the Envrionmental Protection Agency and the Health Service Executive want more tests of the water quality to be conducted before the boil water notice can be lifted.
The tests will continue until Tuesday.
Speaking earlier, Mr Gleenson said: "We've made some improvements to the plant during the week and it is operating well. We have taken a lot of data of how the plant is operating and that is what the EPA will be looking at, along with the two satisfactory samples that we have from the water that is entering the system."
He went on to say that while the plant was currently delivering water to the quality that was needed, the EPA and the Health Service Executive need to see that the quality can be consistently delivered.
Heavy rainfall in recent days has added to problems at the Leixlip Plant.
Mr Gleeson said they have tried to take pressure off the facility by ramping up the Ballymore Eustace plant, which is "running really well".