Feeling as though I hadn't visited as much of underground Sydney as I should have by now, I made the journey to a new drain tonight.
Apparently, this is known by some as 'Deep-Throat', and I can see why. Although it begins as rectangular concrete, it quickly transitions to a strange esophageal shape.
Although there's no reference for height in the photos, I could only just stand up in the rectangular section..
The drain continued like this for as far as i could bear to walk. Perhaps next time I'll bring my bike.