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.

When the state Labor opposition requested M4 traffic volumes from Roads and Maritime Services using freedom of information laws last year, the Sydney Motorway Corporation said the release of traffic volumes would "significantly prejudice negotiations" around the sale of WestConnex.

The government intends to sell the existing, and future, sections of the $16.8 billion WestConnex motorway this year.

This week, Fairfax Media asked government agencies why the release of traffic volumes would prejudice the sale when, presumably, those volumes would be available to potential buyers. No one at Roads and Maritime Services, Sydney Motorway Corporation, or acting WestConnex Minister Melinda Pavey would address the question.

They also did not address the question of how many motorists were using the road.

The Labor leader, Luke Foley, said: "This government is so addicted to secrecy it can't even confirm the most basic facts about WestConnex."

"Every toll operator regularly publishes its figures to investors so why can't this government? It is a cruel irony that the people who are paying the tolls are being kept in the dark."

In November, chief executive of Sydney Motorway Corporation, Dennis Cliche, told an infrastructure conference that about 150,000 vehicles were using the widened section of the M4 on weekdays.

Mr Cliche said that figure represented "a really great story" because fewer motorists were switching to the toll-free Parramatta Road than expected. He said traffic figures had dropped 25 per cent on the M4 since tolling started, compared to a forecast 40 per cent drop.

Mr Cliche's public figures seem to compare well to traffic forecasts made prior to the road's opening. The 2015 business case for WestConnex predicted that 163,800 vehicles a day would use the widened M4 motorway by 2031, well after the entire motorway is open.

Anecdotally, there has been a large increase of traffic on the adjoining Parramatta Road.

A spokesman for RMS said: "Specific traffic volumes are commercial in confidence with Sydney Motorway Corporation subject to divestment."

The spokesman, however, confirmed that "traffic volumes will be provided to potential buyers."

Ms Pavey said: "WestConnex is one of the most important road projects in Sydney and the biggest in Australia today, and will change millions of people's lives for the better."

Traffic volumes on roads owned by Transurban, the country's largest toll-road operator, are published on the Australian Securities Exchange.