SIPTU has said that its members in Irish Water will not accept any unilateral changes to their terms and conditions of employment.
The statement comes after the company confirmed that it would cancel performance-related payments for staff for this year and last.
An independent review is to be commissioned to examine such payments in the future.
SIPTU represents about 100 of the 400 workers directly employed by Irish Water.
The union and other unions will meet company management tomorrow.
Ervia, the parent company to Irish Water, said it had decided not to pay what it calls "performance-related awards" to Irish Water employees for 2013 or this year.
An Ervia spokesperson said the decision reflected the fact that at the end of 2014 Irish Water has not yet earned sufficient public confidence.
The spokesperson said the decision had been made in the best interests of Irish Water and the wider Ervia group to show it was listening to the public.
The company said it had also initiated an independent review of the pay model that applies in Irish Water, which would recommend an appropriate scheme to be applied in the future.
The company also confirmed that an independent review of the bonus scheme will be commissioned.
The issue of executive bonuses or performance pay has plagued Irish Water, with almost a quarter of its staff apparently qualifying.
Ministers hope that this development will improve the public mood as they prepare to announce the revised water charges scheme tomorrow.
Minister for Transport Paschal Donohoe said he has always believed that bonuses should only be paid to people on the basis of exceptional performances.