The cause of contamination in water from a scheme in Mayo has not been found, an audit report from the Environmental Protection Authority has said.
The Drinking Water Audit Report, published on Tuesday morning, said there was "no evidence of a route for a cryptosporidium breakthrough" at the Lough Mask Regional Water Supply Scheme.
A boil water notice, imposed on September 2nd, continues to be in operation for 46,500 people who are supplied by the plant after a sample taken on August 31st, tested positive for cryptosporidium, a parasite that causes gastrointestinal illness.
The EPA carried out an audit of the water scheme on September 6th.
In a previous report, in May 2015, the agency warned there were significant operational issues at Lough Mask treatment plant that presented a risk of “filter breakthrough”, which could result in cryptosporidium entering the water supply.
In the audit published on Tuesday, the EPA said Irish Water had made significant improvements in its filtration management and control since 2015.
“There was no evidence that filter breakthrough had occurred during the two weeks prior to the date of the cryptosporidium detection,” it said.
The audit said the “robustness” of water treatment at Lough Mask could be improved and noted Irish Water was undertaking an upgrade of the plant, due for completion in May 2017, which would further improve the quality of water treated prior to filtration.
It recommended that Irish Water continue daily sampling of water until the boil notice is lifted.
A spokeswoman for Irish Water said the single positive sample of cryptosporidium was “totally unexpected”, as the plant was working well. She said the priority was to keep sampling.
Residents in all major towns in the south of the county, including Castlebar, Westport, Claremorris, Ballinrobe, Ballyhaunis and Knock, are affected, as are the communities of Ballindine, Balla, Kilmaine, Cong and their associated hinterlands.
Some group water schemes are also covered by the boil water notice.
