The state government has traded blows with the Opposition over the extent of the boost to passenger capacity on buses from Sydney's northern beaches to the central city when the $514 million B-Line bus project is completed as early as mid-November.

Labor leader Luke Foley used leaked Infrastructure NSW documents on Tuesday to accuse the government of pursuing a multimillion-dollar bus project that "won't improve things at all" but would need improvements almost as soon as it was opened.

A "health-check" document for the project in May said the B-Line was forecast to boost total capacity for passengers – both seated and standing – by 19 per cent on weekday mornings between 6am and 10am from Sydney's northern beaches to Wynyard in the CBD, but by just 30 passengers during the key period of 8am to 9am.

The Opposition's claims prompted Transport Minister Andrew Constance to accuse Mr Foley of "getting your numbers wrong" by relying on an outdated Infrastructure NSW document.