The month-long toll holiday for motorists on a widened section of Sydney's M4 motorway cost the state government $15 million in compensation payments to the corporation it set up to deliver the WestConnex motorway project.

The Minister for WestConnex, Stuart Ayres, revealed details about the flat fee paid to Sydney Motorway Corporation at a budget estimates hearing, where he was grilled by the Greens and Labor about the impact the reintroduction of tolls on the M4 has had on traffic.

Since the month-long toll holiday ended on August 14, traffic on a 7.5-kilometre section of the M4 between Parramatta and Homebush – widened as part of the first stage of WestConnex – has dropped by about 25 per cent.

It has led to traffic congestion worsening on Parramatta Road and other free alternative roadways as motorists avoid the distance-based tolls of up to $4.56 each way on the M4 for cars.