A new era, or a return to an old era, starts this week on Sydney's public transport system as police begin taking responsibility for security on the city's trains, buses and ferries.

The transport minister, Gladys Berejiklian, and the police minister, Mike Gallagher, joined senior officers this morning in announcing a new NSW Police Transport Command had started operation.

It is not the first time police have been put in charge of Sydney's transport security. They were responsible for transport security before 1998, when the then minister, Carl Scully, hired Chubb security guards to patrol trains.

The Chubb guards were in turn replaced by RailCorp's transit officer division in 2002. That division will now be shrunk from 600 staff to 150 over the next two years, and officers spread across buses and ferries, as well as trains.

On the numbers, the O'Farrell government's changes mean a thinning in the ranks of "revenue protection" officers to check passenger's tickets.