Alrighty. Im digging already. :)
Because of where the plans say the pipe is and as the electrickery needs to run the same way, I have a trench a meter wide by almost two meters deep already. Its a good ten meters from the side of the workshop to the fence, and Im only a foot away from that. Yep, Perth dirt is sandy. Im allergic to shovels now. I havent the access space to get a backhoe in.
Thanks for the suggestions. I have messaged Subsurface Detection, a GPR mob in Perth. I hope they can sort this out, I want my shed in operation.