Irish Water and Fingal County Council have been criticised by a report into last week's 'boil water' notice.
The Environmental Protection Agency said a number of previous recommendations it made last March have still not been implemented by Irish Water.
600,000 people in Dublin, Kildare and Meath were not able to drink from their taps for three days because of problems with the treatment plant in Leixlip, Co Kildare.
The Co Kildare treatment facility is the second-largest in the country.
On Monday night of last week, a number of processing alarms were activated but it took two hours between the first alarm and the plant being shut down.
The EPA said the problem was caused by a blockage in a pump.
As the oversight body, it said Irish Water and Fingal County Council failed to implement recommendations from a previous audit back in March which it said is unacceptable and contributed to last week's issues.
The EPA also said a number of safety recommendations from its March audit that should have been carried out this year will only be implemented between 2020 and 2021.
Irish Water meanwhile has been given a month to respond about how it plans to deal with the issues raised by last week's incident
Green Party MEP Ciarán Cuffe has said action must be taken to ensure this does not happen again.
"I'm not convinced that an incident like this won't repeat itself," said Mr Cuffe.
"It's a very serious issue when water has to be boiled for hundreds of thousands of people.
"And I think we need to improve the quality assurance process all the way from pipe to tap.
"I'd hope that Irish Water will learn lessons from this and that Fingal County Council and other local authorities would learn their lesson also."