Speculation the Baird government is preparing to privatise more of Sydney's mass transit system has been revived after bureaucrats refused to release nearly 100 documents on the subject by declaring them cabinet in confidence.

The refusal raises questions about whether the government, already reaping the benefits of a multi-billion dollar sale of state electricity assets, is planning to change the operation of the state's public transport services.

Labor sought from the government correspondence including reports, briefing notes and emails that related to options to privatise or outsource parts of NSW Trains and Sydney's trains and buses.

Officials at NSW Treasury records found 97 documents relating to privatisation. But they declined to release them under freedom of information laws because of an "overriding public interest against disclosure".