The chief executive of the company building the $16.8 billion WestConnex motorway has described the reintroduction of tolls on the widened M4 as "exciting" while ridiculing concerns about the health impact of exhaust ventilation stacks.

At a conference on Monday, Dennis Cliche also said 17 million toll "transactions" had taken place with about 150,000 vehicles using the 7.5km widened section of the M4 on weekdays.

Mr Cliche confirmed that since the reintroduction of tolls in August there had been a 25 per cent drop in traffic using the widened section of the M4 - stage one of the three stage, 33km WestConnex - pushing more cars onto congested Parramatta Road.

But in his speech to the Infrastructure Partnerships Australia conference, he said the result is "a really great story" because the drop off is far less than the 40 per cent forecast.