The New South Wales Government is considering introducing distance-based tolling across a range of major Sydney roads, including some that are currently free.

Per-kilometre tolling is already used on Sydney's M7 motorway.

The idea of switching to distance-based tolling was first raised earlier this year in a Commission of Audit report by former Sydney Water chief Kerry Schott.

The Government is now holding forums to discuss its long-term Transport Master Plan.

A discussion paper for the plan looks at road pricing and distance-based tolling, and also discusses charging on some parts of the road network that are currently untolled.

The ideas have alarmed Opposition Leader John Robertson, who says it will force up the cost of living for people in Sydney's west.

"One year since the election and finally we're getting a glimpse of Barry O'Farrell's plan to beat the traffic problems in Sydney, and that is to make tolls so expensive that no-one can afford to drive any more," Mr Robertson said.

"Barry O'Farrell was elected on a promise to reduce the cost of living, but one year on he's hatching a plan that's going to cost motorists more.

"If this plan goes ahead drivers on already congested roads like the M4 are going to be forced to pay up to $20 extra each way to travel to the city."

Roads Minister Duncan Gay says the Government inherited a massive infrastructure backlog and it needs to have a sensible discussion with the community about how to pay for enhancements to the road network, now and into the future.

NRMA president Wendy Machin says there should be a debate about road funding, but there would have to be major benefits to justify any tolling increase.

"All Governments need to remember that motorists pay a lot now to use our roads," she said.

"We pay about $14 billion or $15 billion in petrol taxes to the Federal Government and we'd like to see more of that come back to our roads.

"On top of that we pay state taxes and charges and tolls, so if we're going to have a discussion about it we've got to be fair."