Q Gardai are using on-the-spot fines to pay the cost of privately operated GoSafe speed cameras, which are losing €11m a year.
The C&AG said the cost of paying private operators to check speeding drivers was more than €15m in both 2011 and 2012, but that just €4.6m was being collected in fines, resulting in a massive shortfall.
And it says that the Department of Public Expenditure and Reform only sanctioned a spend of €13.5m, excluding VAT, to operate the system.
Garda management are making up the shortfall by using money collected from other fixed-charge penalty notices.
The GoSafe contract was awarded in 2010 and operates for 6,725 hours a month.
The C&AG said last year it cost €15.6m to operate the system, with just €4.6m collected in fines.
"An Garda Siochana financed the shortfall through the retention of receipts from fixed-charge notices issued on foot of other enforcement activities," it said.
Garda Commissioner Martin Callinan told the C&AG there was no funding allocated in the annual garda estimate for the cameras, and that it was decided to "ringfence revenues" to pay the bill.
He added that permission would be sought from the Department of Public Expenditure and Reform to retain receipts from fixed-charge notices to fund the GoSafe system.