30 year old house in Western Australia (presumably) built on a concrete slab on top of sand .
The drainage pipe from the shower to the main clay sewer pipe suffers from periodic slow flow and the visible end where it exits the house looks very rusty (layers of rust peeling away). I suspect that the builder used galvanised steel instead of plastic pipe and it's collapsing internally.
I have no idea how thick the concreate slab is, but the shower floor is recessed 5cm (2 inches) below the main floor level. The shower drain hole is 2.6m (102 inches) from the outside wall.
What would I expect to find if I tried to drill/cut around the existing drain hole through the floor? Would the existing pipe be totally encased within the concrete slab? Is there a standard technique for doing this kind of pipe replacement operation?