THE Government expects that at least one in five householders won't register for water charges even with the lower bills.
The coalition hope that the capped fee of €160 will neutralise most of the opposition to the charges.
However, while the level of registration for water charges is viewed as crucial, only half of householders had signed up before the announcement of the new charging system.
And there is acceptance within the Coalition that there will still be widespread evasion.
"Anything that beats 70pc [registration] isn't bad at all. But with time, it will get down [the non-payers] to under 20pc," a senior Government source said.
Meanwhile, administration of the €100 water conservation grant risks compounding already excessive delays in welfare payments for some of the poorest people in the country, an opposition TD has claimed.
The grant, to be administered by the Department of Social Protection, brings the overall sums paid by householders for water down to €60 for a single person and €160 for households with two adults or more.
Social Protection Department statistics show that certain kinds of welfare payments can take several months - and in some cases well over a year - to conclude if the case is subject to an appeal. For instance, the figures show that someone claiming a carers' allowance is waiting an average of 32 weeks for a decision. Fianna Fail's social welfare spokesman, Willie O'Dea, who received the statistics in response to a Dail question, said the decision to pay the water charge grant via the Social Protection Department was very bad news for some vulnerable people.
"The huge administrative burden being put on the Department of Social Protection by designating it as the paying agent for Water Conservation Grants will have further impact on the poorest and most needy people in the country.
"It means that the staff in Social Protection, which is not being increased, will take even longer to determine a person's entitlement to a basic benefit.
"It is outrageous to do something to further lengthen those waiting periods, because in most cases the applicants will have no income and will be surviving on the charity of relatives - many of whom can ill afford it. However, that is the inevitable consequence of loading this extra burden onto the Department," Mr O'Dea added.
An official at the Social Protection Department said it was too early to comment on what extra staff, if any, would be made available to process the new €100 grant.
"The Department of Social Protection is currently assessing its requirements to administer this payment," the official said. The department statistics show a wide variation in processing times. For example, the standard unemployment payment, known as jobseeker's benefit, averages one week, while a contributory old age pension can be processed in an average of two weeks. The most time-consuming are disability allowance (30 weeks); carers' allowance (35 weeks); and invalidity pension (25 weeks).
The anti-water charges campaign is also urging workers to take a day from their holiday leave rather than sacrifice their pay to attend the next national water charges protest on Wednesday December 10.
Socialist Party TD Ruth Coppinger has suggested the next demonstration might become an unofficial national strike, which would mean protesters' wages will be deducted.
The fact the protest is being held on a work day - unlike the previous two weekend marches - may affect the turnout.
Employers will dock a day's pay if staff are absent without permission. But Right2Water organisers said there was no need for workers to give up their wages to take part.
John Douglas, organiser and General Secretary of the retail union, Mandate, said taking a day's annual leave was the best option. "We're advising all of our members to attend where possible," he said.