The cost to NSW taxpayers of a scheme that refunds tolls to motorists who use the M5 motorway in Sydney's south-west is set to top $2 billion within the next two years.

Combined with the state government's rego rebate for users of toll roads, taxpayers will be on the hook for more than $240 million annually over the next three years in toll subsidies.

The annual bill for the state of the cashback scheme for privately registered motorists driving on the M5 South West has doubled in the past decade to about $115 million in the 12 months to June.

The cost to the government of the M5 cashback scheme was about $115 million last financial year. Credit:Photo: Brendan Esposito

An analysis of figures from the state's transport agency shows motorists have received more than $1.7 billion in rebates since the cashback scheme was rolled out by the Carr Labor government in the late 1990s, which included a now expired refund for tolls on the M4 motorway in the city's west.