The Berejiklian government will tell investors, but not the public, how many motorists are using the first section of the WestConnex motorway.

Tolls were reintroduced on the M4 motorway in August, a month after the widening of the road to four lanes in each direction was completed.

But the only indication of traffic volumes on the 7.5-kilometre widened section has come in limited public comments by the chief executive of the Sydney Motorway Corporation, the "private" entity established by the government to build and run the motorway.

The government is refusing to provide any further information about traffic volumes on the M4, widened at a cost of about $500 million.