If you can find the entry into the catchpit or manhole you could guestmate the fall up from that. The best way is to get a drainlayer with the detection equipment to find it's exact location and depth. This can be expensive if an 'A' frame etc is required for the entry into the man hole. Mine was about 6m deep so needed 2 guys and harness etc. (just to confirm they didn't find anything 😀)

Contact these guys. https://www.beforeudig.co.nz/nz/home

They will help you find the locations for all services etc installed on a site.

Varkk:

 

Contact these guys. https://www.beforeudig.co.nz/nz/home

 

They will help you find the locations for all services etc installed on a site.

 

Just watch this service. While 'Beforeudig' don't charge, some of the retailers & service providers do charge. Also i'm not sure they do stormwater, if they did it would be through a service you could hire yourself.

kiwis:

 

Are there any experts around who can tell me how I find out how deep a stormwater drain is on my property. Council ones not private

 

I don't know where you are, but in Hamilton you just ask the council. They have as-built plans of all their infrastructure. I used to work in that department.