Thousands of commuters in Sydney are taking public transport for free today as bus drivers escalate their industrial action against the Berejiklian Government's plan to privatise some services.

Bus drivers from 12 depots around the city declared a "fare-free day" on Thursday and turned off their Opal Card machines, meaning passengers could not pay to ride.

The Rail, Tram and Bus Union said drivers were worried contracts covering services at other depots may be also be dishonoured.

The union's divisional secretary Chris Preston said drivers were furious.

"They're out there trying to let the public know what Minister [Andrew] Constance is up to because the public overwhelmingly do not want their buses privatised. They want to keep them in public hands," he said.

Declaring a "fare-free day" was the latest in a string of industrial action by drivers.

Last month, about 1,200 drivers around the inner west walked off the job for 24 hours.

Buses filled NSW Transport's Tempe depot as up to 1,200 bus drivers went on strike last month. ( AAP: Dean Lewins )

Lane Cove woman Samantha Samuels catches the bus every day and said skipping the fare was a nice surprise.

"The trip to my office in Darling Harbour normally costs $3.50 one way so yeah, it was good not to pay," she said.

The Government has said the decision to privatise some services was made after a deluge of complaints.

Mr Constance said the Government was not selling buses or depots and would provide five-year job guarantees for drivers affected by the plans.

Passenger Raj Sagh said he supported the bus drivers' decision to let passengers ride free, and he was worried fares would go up if bus services were privatised.