Melbourne's second biggest bus operator has been ordered to take a dozen of its buses off the road due to serious defects that posed a danger to passengers.

A blitz by safety inspectors on two Transdev bus depots found 33 defective buses, with 12 in such poor condition they were ordered off the road for urgent repairs.

A Transdev bus on Lonsdale Street. Many of the company's buses have been deemed unroadworthy. Credit:Daniel Pockett

Victoria's transport safety watchdog, Transport Safety Victoria, said it was the highest number of defective buses it had ever taken off the road in a blitz.

Transdev, which operates one-third of Melbourne's bus network under a $1.7 billion contract with the state government, has been forced to hire a mix of buses and coaches from other companies to plug holes in its timetables.