Today is the registration deadline for Irish Water customers who want to avail of the Government's €100 Water Conservation Grant.
As of last week, Irish Water said roughly three out of 10 households still have not signed up.
The utility said it is expecting a last-minute rush to register.
Anti-water charge campaigners are urging even those who have already registered not to pay their water bills.
Anti-Austerity Alliance TD Ruth Coppinger said that a boycott could force the next Government to scrap Irish Water.
"Within the next nine months, there will have to be a new Government in place," she said.
"That Government will come under huge pressure is a huge number of people have been boycotting the bills, it will put them under pressure to abolish the hated Irish Water and the hated water tax."