People Before Profit is warning there will be more water protests if the Government tries to change the free allowance rules.
It comes after a new utility report shows that 7% of homes are responsible for around a third of all water usage.
Under the current plans, only those who use excessive amounts will be charged.
Richard Boyd Barrett TD is warning there will be trouble if they attempt to change that.
"While most people would not face charges - initially, at least - because the allowance has been given would not be used, those thresholds could always be varied in the future to try and essentially bring charges in through the back door," he said.