The Managing Director of Irish Water has said it could be a number of days before the latest boil water notice is lifted.
It is the second notice in the last two weeks for people living in parts of counties Dublin, Kildare and Meath, with 615,539 people affected.
The issue has arisen as a result of weather over the weekend affecting water quality at the Leixlip Water Treatment Plant.
The problem has resulted from very heavy rainfall that has washed large amounts of organic matter into reservoirs, increasing the turbidity - or 'cloudiness' - of the source water above acceptable levels.
The older of two water treatment plants at the Leixlip facility has been unable to react fast enough to deal with the rise in turbidity.
Irish Water says there are currently 18 boil water notices in place across the country, including Leixlip, affecting more than 629,654 people.
They include the Lough Talt regional water supply, with 12,576 customers affected in parts of Sligo and Mayo, Tallanstown in Co Louth with 588 people affected, and the Galtee regional water supply with 402 people affected in parts of Tipperary.
Speaking on RTÉ's Morning Ireland, Niall Gleeson said Irish Water was working with the Environmental Protection Agency and the Health Service Executive to establish criteria for lifting the Leixlip notice.
He said it was likely there would not be any change today.
"We need two or three samples, so that will take a number of days. We're working with the HSE and the EPA to address the criteria," he said.
Mr Gleeson said there are two plants in Leixlip, one of which was built about 40 years ago.
He said this was the plant that had been causing problems and the company was in the process of refurbishing it.
He said that it supplies about 20% of the water to Dublin.
"While we are refurbishing it, we can't take it out of service," Mr Gleeson said.
"Ideally we would shut the plant down for six to 12 months and do all these refurbishment works, but because it's such a critical part of the Dublin water supply, we have to keep it in operation," he said.
Mr Gleeson said the circumstances between this boil water notice and the notice issued two weeks ago were "quite different".
He said the first issue was related to operational issues, while operators caught the problem in the second issue and purposely shut down the plant.
The current problem involves issues in the treatment process at the plant that may have compromised the disinfection process, which makes the water safe to drink.
Mr Gleeson said Irish Water had a choice of either restricting water supply, which would leave people unable to flush toilets or have showers, or introduce the boil water notice.
He said that what happened over the weekend demonstrates that the plant can run "effectively and efficiently", and that refurbishment takes time.
"It's a bit like trying to change the tyres on a car that's running down the road. We can't take it out of service," he said.
This afternoon, Mr Gleeson told the Committee on Housing, Planning and Local Government that the water treatment plant in Leixlip is still not delivering satisfactory levels of treated water.
Mr Gleeson told the committee that they did consider delivering water by tankers in the areas affected by boil water notices.
However, he said the HSE recommended that water from tankers should also be boiled, so he said there was no advantage in doing this.
Those in affected areas must boil water used for drinking, the preparation of salads, brushing teeth and making ice.
Further information can be found on Irish Water's website, or customers can contact the helpline on 1850 278 278.
The Director of the EPA Tom Ryan has told the committee that there has been a deterioration in the infrastructure treating the plant.
He said the use of ultra violet disinfection is the right way to go and had been recommended in recent audits.
Mr Ryan said an automatic shutdown on the entire plant had been recommended by the EPA but had only been implemented on one of the lines last month.
He said there was a period of time when the plant was operating when alarms were ringing.
Mr Ryan said precautionary measures were taken including flushing out the system before the boiled water notice was lifted.
Fine Gael TD for Dublin North West Noel Rock said it was very frustrating that the boil water notice had reared its head again, so soon after the last incident.
He said people want to know when water will be restored and to be reassured it will not happen again.
RISE TD for Dublin West Paul Murphy said it was scandalous that Irish Water appeared incapable of providing clean drinking water to people and there must be sanctions for failure to deliver EPA recommendations.
He said funding for Irish Water was inadequate and that high earners and wealthy should be taxed at higher levels in order to fund it.
Additional reporting Aisling Kenny