Hi Liliana,
I have just gone over some other threads and PM's and now think the following.
Your water tank overflow discharges to a 100 mm DWV (storm water system) pipe. This we already knew.
The hot water service discharges overflow through a vertical pipe going below ground. This has been the cause of past confusion.
You have a 90 mm surface drain pipe as per the photo in this thread. This has to connect to a 100 mm DWV (storm water drain) pipe.
The next logical assumption is to assume that the surface drain pipe AND the vertical drain pipe at the HWS AND the water tank overflow pipe all connect to the same underground drain pipe (storm water system).
This pipe is not shown on your plans (hence the confusion) and has been poorly explained by the builder to you as per other correspondence.
The storm water pipe is separate to the wet pipe.