When a vehicle strikes a kerb, it can act like a ramp. The trajectory of the vehicle will depend on the speed, angle, kerb dimensions, vehicle characteristics and more. Upon striking a kerb at speed, an airborne vehicle has the potential to vault an adjacent barrier or underrun a barrier upon landing.
The Victorian DoT has recently modified Zone 1 and 2, and has removed Zone 3 from its guidance. The new guidance can be found in the DoT Supplement to AGRD Part 6: Roadsides, Safety and Barriers V4.0, Aug 2021.
Snip of the new Table V5.3 below.