The Simon Communities say there were no homes available to rent under the HAP scheme in Waterford City last month.
They are calling for Housing Assistance Payment rates to be increased.
A new report by the homelessness charity found only 80 properties were available to rent under the scheme last month across 16 urban areas.
The 'Locked Out' report for March also found that 10 areas it studied had no rental properties available under HAP rates, including Waterford City.
There were 12 properties in the city listed to rent generally but none of them were available under HAP.
Wayne Stanley, Head of Policy and Communications at the Simon Communities of Ireland, says current HAP rates were set more than five years ago:
"It hasn't increased since 2016 and we know that rents across Ireland have increased by more than 44% in that time.
"The HAP rates are just not at the level that supports people and particularly at the moment, when we're seeing this cost of living crisis following behind."
Continuing, Wayne says the rates simply don't cover the current market rent:
"What the HAP rate scheme being so far behind market rents means is that people are having to top-up from their own income and, in the context of all the basics - bread, milk, heating - all increasing exponentially at the moment by huge percentages.
"What that means is that people are having to make the decision between food and rent and that just isn't sustainable.
"It just pushes people, who are in relatively secure accommodation when it's rented accommodation, it pushes them towards homelessness."
