Thank you so much for answering these questions!
I’m actually in Melbourne (but am a NZ’er at heart).
I will try and get the plumber to chat today.

Sorry, I wrote the last post not particularly clear. The tank itself is above ground, but all the down pipes etc feed into it via pipes that are underground.
I really…

Thanks.
So am I right in guessing that the grey DWV pipes are more expensive than the white stormwater?
You mention that black isn’t common for charged systems - is there…

Looking for advice please - and apologies, I don't know much about plumbing requirements.
We have had an extension/renovation to our home and have had to install a charged system…