Sydney on Tuesday joined London, Singapore and other big cities in charging motorists for driving into the central business district (CBD) at peak times.

But the New South Wales state government is denying that a toll levied on cars coming over the famous Harbour Bridge into the city that varies according to the time of day is a congestion tax.

"This is not a congestion tax, it's nothing like a congestion tax, this is time-of-day tolling," Treasurer Eric Roozendaal said.

"On a congestion tax, whether it be Singapore or London, all access to a particular part of the CBD has a very high charge on it during the peaks to discourage people coming in at all," the state's finance minister added, insisting the government had not broken an earlier promise not to introduce a congestion tax.

But motorists were as one in describing the levy as a de-facto congestion tax. The tax, which replaces a previous 2-Australian-dollar (1.34-US-dollar) toll for crossing the Harbour Bridge, varies from 4 dollars in peak times to 2.5 dollars off-peak.