More than 58,000 households are to be billed for using too much water.
Irish Water has begun sending out notification letters to thousands of users.
Internal Irish Water documents, obtained by the Irish Daily Mail under the Freedom of Information Act, show how many households will be affected by the excessive use charge.
Irish Water has started sending out notification letters to 58,000 people that it believes are using too much water.
Water use above the annual household allowance of 213,000 litres is considered to be excess use.
Irish Water says 213,000 litres would almost fill an Olympic-sized swimming pool.
But households with more than four people are entitled to a higher allowance of 25,000 litres per person.
Customers with medical needs who require additional water can also apply for an exemption from excess use charges.
The utility has warned people they will be liable for their excess use from the second half of the year - unless they reduce consumption.
Households that do not cut down on water use will be billed from January 2022.
The excess will be charged at a rate of €1.85 per 1,000 litres.
This will be up to a maximum of €250 for water coming in and another €250 for water going out.
There will be combined cap on charges at €500 for water and wastewater services.