I do not know if that is the official name for it as me and my mates used to call it a different name (very similar to blackwater), which I do not want to share as it may give away the area. I was told by someone years ago that this was the name of the drain.
It is a Sydney drain that was accessible via water, however I do not know if I need a manhole key these days as works have gone on in the area. I accessed it as a kid when I was exploring with my friends. We liked to do those sorts of things when we went to the inner-west.
That being said I do not know if it is a full drain system as I did not fully explore it as a kid.