Fianna Fail says the costs of setting up Irish Water as "ridiculous and abhorrent". It is responding to claims that the utility will spend €2 billion paying unnecessary staff being transferred from local authorities.
Fianna Fáil environment spokesman Barry Cowen says the government cannot keep deflecting questions about how much households will pay.
He says the coalition has not learned from its own previous mistakes.
Speaking in Brussels this morning, the Finance Minister Michael Noonan said he expects staff numbers at Irish Water to fall by 1,000 after all water metres have been installed.
The Taoiseach meanwhile has denied taxpayers will end up paying more for water as a result of set up and staff costs at Irish Water.
The ESRI says Irish Water needs about 1,700 staff, but agreements with local authorities mean it will actually be paying over 4,000 people.
Enda Kenny says the utility company will help reduce the cost of water in the long-run.
Professor John Fitzgerald of the ESRI has argued that the agency will be paying between €1 and €2 billion more than neccesary. He claims the overspend will mean more expensive water bills for the public.
Professor John Fitzgerald of the ESRI told The Pat Kenny Show here on newstalk that 4,000 staff is about 2,500 more than Irish Water actually needs. The overspend on staff is the latest spending controversy to hit Irish Water, after previous rows over the money paid out to consultants and the staff bonus scheme.
It has also emerged that 29 staff recruited by Irish Water are earning over €100,000.