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".

The documents were declared both cabinet-in-confidence and also likely to impact the government's commercial interests and have "ramifications beyond the public sector".

Unions fear the Baird government intends to put more state-run services in Sydney in the hands of private operators. Their concerns mounted in November when the government unveiled plans for private companies to run Newcastle's buses and ferries, as well as the city's light rail line once it is built.

Shadow treasurer Ryan Park, who made the request, seized on the refusal to release the documents as evidence the government was giving serious consideration to a proposal.

"This government must come clean about whether or not they intend on selling Sydney Trains," he said. "It is time for the secrecy to end and to tell the community and workers exactly what is going on."

A spokesman for Transport Minister Andrew Constance said there "are currently no changes to the way services are to be operated".

"NSW has a dynamic mix of public transport services offered through a range of different models, which has been long-standing practice," he said. "Any inquiries relating to a [request under freedom of information law] submitted to Treasury is a matter for Treasury."

Alex Claassens, the NSW secretary of the Rail, Trains and Bus Union, said privatisation had increased transport costs in Victoria.

"The reality of life under privatisation is that the public never stops paying a price to the private providers," he said. "Selling off our train network would result in fewer jobs, a decrease in services, and an increased cost to commuters."

While private companies such as Hillsbus operate some runs in Sydney, the government-owned State Transit provides the lion's share of regular bus services. However, the government has left open the possibility of a private firm running the so-called rapid buses between the CBD and the northern beaches when they begin late next year.

The Tourism and Transport Forum has long pushed for Sydney's bus network to be placed in the hands of private operators.

The lobby group released a report in February that estimated the privatisation of the services could lead to savings of almost $500 million over five years.

While state-owned Sydney Trains runs the city's suburban rail network, Hong Kong developer MTR Corporation will operate the first stage of Sydney's new metro line from the northwest to Chatswood when it opens in early 2019.

MTR is also expected to be the leading candidate to run the second stage of the line from Chatswood to Bankstown via Sydenham, which is due for completion by 2024.

This will mean that two existing lines run by Sydney Trains – the Epping to Chatswood Line and the Bankstown Line – will, on current plans, be privatised.

The private sector already runs Sydney Ferries, and the light rail line between Central Station and Dulwich Hill in the city's inner west. In February, the state's acting Auditor-General, Tony Whitfield, found the decision to privatise Sydney Ferries in 2012 was justified, citing about $200 million in savings and a good service record.