Until now, only big investors have been privy to the Berejiklian government's figures on how many motorists are using the first section of the WestConnex toll road.

But a leaked document containing weekly data for Sydney's widened M4 shows traffic dropped by 32 per cent in the first full week of tolls to about 942,000 trips, compared with average volumes in the weeks before the charges were introduced in August.

About 1 million vehicles travelled on the 7.5 kilometre-section of the M4 between Parramatta and Homebush in the first week of November, up 6 per cent on the first full week of tolls in August but still down by 28 per cent on the weeks before they began.

Aside from limited public comments, the government has refused to release detailed information on weekly traffic, saying it would "significantly prejudice negotiations" around the sale of a 51 per cent stake in WestConnex.