Timing belt service

I own a Toyota Fortuner 2.5 D4D. I have been doing the services regularly with Toyota. According to the service agents, the timing chain is due to be replaced on 150000 km. I recently did the 140000 km service but unfortunately the timing belt broke while the vehicle is on 145000 km. This has caused major damage to the engine which is now going to cost an arm and a leg to repair. Is this an oversight on the part of Toyota South Africa. Has this happened to anyone else out there?