As cyril7 has pointed out, the diagram getting bandied about is for developers. Wanting to install conduit, in preparation for the Chorus Fibre installer to come and haul microduct through. It needs to be at a minimum 450mm depth because these types of conduit are actually very week and susceptible to compression damage.
The purpose of shallow burying or surface mounting the cable is to:
A) keep it from getting bent at sharp angles which would damage the Fibre inside.
B) keep it out of the way so its not a trip hazard.
The point of burying is not to protect the cable from spades. That is the purpose of the microduct itself, which is very much stronger than people think. It takes a concerted effort to cut through. More so than the existing copper leadins which are usually just direct buried as well.
Remember we are offering a basic install for free. You are more than welcome to dig your own trench or pay someone else to do so, if you want the cable buried further than the 200mm we specify. Simply agree with the technician where you want the cable to run, have the trench dug, then book for the tech to return and lay the cable.