Does anyone know how close to a neighbours boundary / fence line that the underground mains supply cable is legally allowed to be?

Background: Friend has had a new house built next door and the neutral screened mains cable is actually on the boundary - such that digging the boundary post holes is impossible.

(It was hit trying to dig a 600mm hole - no warning tape either! And YES it is live - the people have moved in)... The electrician says it IS legal!

Any help appreciated...I'm loathed to pay almost $200 for a copy of the regulations.

Thanks
Murray