Irish Water says it will be Tuesday at the earliest before a boil water notice affecting more than 600,000 people in parts of Dublin, Kildare and Meath will be lifted.
The notice - which has been in place since Monday evening - is the second that's been issued for consumers of the Leixlip Water Treatment Plant in the past two weeks.
A test carried out on the water quality today returned satisfactory, Dublin Live reports.
However, Irish Water say the Envrionmental Protection Agency and the Health Service Executive want further tests before the boil water notice can be lifted.
The water quality tests will continue until Tuesday.
Earlier, Irish Water boss Niall Gleeson said it couldn't rule out another boil water notice being issued for customers of the Leixlip Water Treatment Plant.
He told RTÉ's Morning Ireland that they need to make sure public health is the driving factor and so "unfortunately the boil water notice has to stay in place for the moment".
He said that while they are trying to do all they can to ensure the safety of the water for customers, there may be more boil water notices issued.
Mr Gleeson said: "Given the condition of the plant and what we are seeing now, there is a possibility of another boil water notice.
"But as I say, we have our engineers on site and we are looking at the plant, getting it back in service this week, but also looking at the longer term.
"Can we add functionality to the plant so we can help it ride through these storms, and then bring it back on line when these storms have passed?
"Pumping up the reservoir is maximising the other plants. So we are doing everything we can."
It is the second boil water notice in two weeks and Mr Gleeson said they realise the impact this is having on so many people and apologised for it.
He added: "I would like to apologise personally, it's a major inconvenience.
"Today we have the EPA on site with the HSE, our own engineers and Fingal engineers are on site as well.
"We've made quite a few improvements to the plant during the week and it's operating well.
"We have taken a lot of data of how the plant is operated this week, and that's what the EPA will be looking at, along with the two satisfactory samples that we have from the water that is entering the system.
"Right now the plant is delivering the water to the quality that we need, but the EPA and the HSE will need to see that we can consistently deliver that quality, and they will need to look at historical records of how the plant is performing during the week."
The warnings also lead to panic-buying of bottled water in supermarkets, seeing shelves empty within hours.
