The NSW government is opposed to limiting increases in tolls on Sydney's growing labyrinth of motorways to the rate of inflation, or extending the pricing regulator's powers to oversee the charges imposed on motorists.

Amid a looming backlash against the reintroduction of tolls on the M4 motorway, the government's transport agency has warned that limiting quarterly rises in tolls to the consumer price index "could have the effect of pushing up the initial price of future toll roads".

"A CPI cap would also make it more difficult to attract private investment; reduce competition in the market; and would be likely to increase costs for government, taxpayers and road users," Transport for NSW has told an inquiry into tolls.

Charges for cars on some of Sydney's largest motorways such as the M2 are, due to contracts signed with private tolling companies, growing at almost three times the present annual inflation rate of 1.5 per cent.