A pipeline drawing water from the Shannon is the only solution to the threat of drought and taps running dry in the east of Ireland, the head of Irish Water has warned.
With the utility set to publish a report on the ambitious scheme in April, Jerry Grant told politicians that close to half of what flowed through the River Liffey everyday was supplied to properties around Dublin.
"The Liffey is currently providing 40% of all its water rescources to meet the needs of the greater Dublin area," Mr Grant said.
"It is absolutely at its sustainable limit. If we had a dry period, a critical dry period, we would be in serious trouble in the greater Dublin area."
Under scrutiny at the Oireachtas Housing Committee, Mr Grant said the imperative of the Shannon water supply project was compelling and growing more acute by the day.
He added that the leakage in the greater Dublin area at about 36-37% or about 200 million litres was a massive challenge.
Irish Water was replacing 70 kilometres of mains in the greater Dublin area as part of an intensive find and fix programme.
Mr Grant said: "There is no quick way out of this other than to build a new scheme that will secure future supply. That is the Shannon scheme."
The committee also grilled Irish Water reresentatives on plans to take complete control of the network through the public utility for more than 1.5 million households by 2019.
It is expected to save €70 million a year but lead to 1,000 job losses in local councils.
The utility's bosses defended plans to introduce an excessive use charge against claims it would represent blantant discriminaton against lower income families.
The accusation was made by Opposition TD Mick Barry who said the introduction of such a charge, expected to be brought in next year, would essentially be the reintroduction of water charges by the back door.
"I want to make it clear that if there are attempts to introduce water charges by the back door they will be resisted not only by political parties like my own but more importantly by communities across the country," the Solidarity-People Before Profit TD said.
It is believed Irish Water plans to introduce a €17 charge for householders who wish to query their bill. It is understood that the charge would be waived if the query proved to be correct but increased to €100 if not.
Mr Barry said: "That is blatant discrimination against low income familes, households and communities.
"Of course the more affluent people will feel confident about querying a bill. How can a low income household feel confident about querying a bill when there's a threat of a €100 penalty over their heads. It's blatant discrimation."
Irish Water managing director Jerry Grant said he was restricted from commenting as the Commission for Regulation of Utilites was considering the implementation of the charge.
The committee was told 80,000 people have been notified of leaks.
Of 4,000 houses that Irish Water is in contact with, each house on average uses the same water as 35 houses.
"There are every practical reasons why we are pursing leakage on the customer side just as we are on the public side," he said.
The introduction of a water charge for households caused huge controversy and led to it being scrapped in early 2017.