Over a quarter of tenants getting state rent subsidies have to make extra top up payments to private landlords.
The Housing Assistance Payment scheme gives state support to those struggling in the private rented sector.
But there has been criticism that the payment rates are too low, as rent prices have surged in recent years.
Comptroller and auditor general, Seamus McCarthy, says in many cases, the state's payments aren't enough.
"The examination found that while market rental costs were increasing, the maximum rents payable by the State through HAP had not been revised since 2017.
"Some tenants made top-up payments to cover any difference in the rent payable to the landlord and the maximum rent payable by the State.
"Around 28% of HAP tenants were making top-up payments to landlords in the first half of 2019."
Department of housing officials say a report on the issue was finally finished before Christmas, and new recommendations are expected shortly.