Be very careful when going down - if you hit unexpected soils or rock it could cost a fortune in extra excavation fees and so on. I suggest getting a Geotechnical engineers report before anything else to make sure it is feasible to build down.

Stormwater is supposed to discharge via stormwater pipe. PVC pipe is superior to clay pipe. Clay pipe is prone to cracking and tree roots and leaks. PVC should not leak. Check all the joins blue glued. Check if glued and still joined together. Check pipe has fall to point of discharge. It could be b...